I won't claim that Yeek is the optimal race for an archmage. It's certainly not the easiest race and class to play, particularly in the early- and mid-game; if you're first starting out, I'd recommend looking at an alchemist, skeleton archmage, or corrupter instead.
Still, a Yeek archmage plays very differently from most other mage builds, and offers some unique advantages:
1) Ultimate, skill-testing, levels of control. ToME is one of the highest-variance roguelikes I've played: no matter how paranoid and well-prepared you are, there will be situations where you die and there is nothing whatsoever you could have done to prevent it. Playing a Yeek archmage comes as close as possible to eliminating these situations. To start with, the archmage class gives you a huge toolbox of talents, with good options for mobility, healing, debuffing enemies, and, of course, dealing damage. The archmage Temporal tree, combined with the Yeek's global speed bonus, stack for a total of 71% increased global speed -- this means you will usually act twice for every enemy turn, giving you plenty of opportunities to use these talents to handle or escape dangerous situations. Carelessness will still get you killed, but no matter what happened, there was probably some way you could have saved yourself with better play.
2) Great racial talents. Unlike any other race, all of the Yeek's racial talents are useful throughout the game, and better yet scale with Willpower, a stat you were going to invest in anyway as an archmage.
3) Outrun orc patrols. Your global speed carries over to the overworld map, so you can dodge most zigur/orc patrols. Definitely the best perk on this list
4) Girlfriend. Skeletons wildfire archmages are probably the most OP archmage build. But they are forever alone.
There are also disadvantages:
1) Little margin of error. With maximized CON and as much +life gear as you can find, you still likely won't have much more than 800 HP by the end of the game. That's... not a lot. In the late game, surprisingly, you'll still be able to clear most content, including High Peak and Farportal bosses, with little difficulty -- but don't expect to be able to faceroll Atamathon and survive.
2) Extremely tough early-game. The Yeek starter dungeons are notoriously difficult. You're playing one of the best classes for clearing them -- but still, expect to die a lot at the beginning. On the bright side, if you can clear the first three dungeons, the hardest part of the game is behind you.
3) Boredom. Thanks to the Yeek exp bonus, you will hit level 50 well before the Charred Scar. Once you do, the biggest danger to your character will be falling asleep and holding down the '5' key with your forehead, as you slowly mop up the rest of the prides and High Peak floors.
Build Strategy: Bird's Eye View
The key to the Yeek archmage build is the Temporal category, and in particular, Essence of Speed. While getting this talent to level 5 is one of your biggest milestones, there are some complications: Essence of Speed sits at the far right of a locked category, and demands a whopping 250 mana to sustain. This same mana is the fuel for all of your damage and escape talents, of course. There is also the matter of survivability: even dumping all of your skill points into CON, your low health pool will leave you a sitting duck for random archers, corruptors, oozemancers, etc. by mid-game unless you supplement your health pool with the Aegis tree talents.
So your game plan, roughly in chronological order, is as follows:
1) Assemble the tri-beam
Manathrust, Lightning, and Flame are the bread and butter of every early-game archmage, and you are no exception. However, unlike most archmage builds, it's a perfectly good idea to invest the full 5 points in these skills -- once you sustain Essence of Speed, you'll be in need of reliable, low-cooldown damage spells to spam, and these fit the bill perfectly.
2) Max the Aegis sustains
Yes, going for a 250-mana sustain does not exactly leave you champing at the bit to give away more mana -- but the two Aegis sustains are indispensable. Arcane Shield turns every heal into a damage shield. At *worst* this increases the power of your healing spells and infusions by 66%. In practice, your healing spells will restore far more than your maximum HP, giving you a huge temporary shield. Combined with Shielding, which passively buffs the durability of all of your shields, and Aegis, which buffs them *again*, it won't be uncommon for each of your healing talents to "heal" you for 5000 effective HP by the end of the game. Max Arcane Shield and Shielding ASAP, and keep them on.
3) Activate Essence of Speed
Unlock the Temporal tree and start putting points in Essence of Speed. There is no rush, really, since activating Essence of Speed with anything less than 500 mana is going to be miserable, as you'll always be running short of mana. Once you can sustain Essence of Speed and still comfortably spam your spells, you have passed a turning point and game difficulty is all downhill from there.
4) Build up the Wildfire tree
Now that you've taken care of survivability and flexibility, it is time to invest the rest of your points in DPS. The two "classic" good choices here are the arcane and fire lines of spells. I recommend fire for three reasons: first, to be effective, an arcane build needs to unlock both the Aether and Meta trees, while a fire build can get away with only Wildfire, and you don't have an overabundance of either category or class points. Second, while the arcane talents will let you dish out higher overall DPS, Wildfire gives you Cleansing Flames, an excellent talent for both defense and offense; the closest thing arcane can do is far less reliable Meta/Disperse Magic. Lastly, a large part of the appeal of going arcane is the use of Disruption Shield, which sadly does not play well with your tiny mana pool.
That said, the arcane and aether spells are fine talents and a Yeek archmage arcane build should be perfectly viable, if you don't to go Wildfire, or can't due to missing unlocks.
Category Points
Per the above, I recommend spending your category points as follows:
1. Temporal
2. Inscription slot
3. Wildfire
4. Inscription slot
There is quite a bit of flexibility here; in particular, spending the last category point on an inscription slot is largely a matter of my taste. Although I won't cover it here, unlocking Meta or Celestial/Light instead are also perfectly good options.
Skill Points
Skill point allocation is quite straightforward.
STR: Useless
DEX: Useless
CON: Even *with* a ton of CON, you will have a pitiful amount of HP. Skimping on CON is suicidal.
MAG: Boosts your spellpower, which scales up most of your talents including your damage, healing, and shielding spells. Obviously you want this as high as possible.
WIL: Determines the size of your mana pool. You need a ton of WIL if you plan on activating all of your mana-hungry sustained talents, and still have something left over for your damage spells.
CUN: Boosts spell crit chance slightly. Nice to have, but not essential to the build.
So I recommend spreading points evenly between CON, MAG, and WIL until they are maxed, then dumping the rest in CUN. If you find the early game too frustrating, diverting some points from WIL to CON in the beginning is perfectly fine. You will need plenty of WIL eventually, though, before you'll be able to comfortably afford Essence of Speed.
Runes/Infusions
I use the following loadout:
1. Rune of Reflection
2. Rune of the Rift
3. Wild (mental & physical)
4. Healing
5. Healing
You want at least one shielding rune. Before you've maxed Arcane Shield, these are your best way of protecting yourself against burst damage. Late game, you might as well use healing runes instead, but the Rune of Reflection is an exception: with Shielding and Aegis you will eventually be able to put up a reflection shield with upwards of 2000 hit points. That's right -- any enemy that wants to kill you will first need to hit themselves for 2000+ damage. This is a fine talent to configure with "auto-use when enemy sighted," by the way.
Rune of the Rift is amazing. First, since you're investing in WIL it hits for very respectable damage: 600-700 by the end of the game. Second, it does so for no mana, making it a great emergency talent if you find yourself stunned, silenced, or out of mana and standing right next to a dangerous enemy; or even if you are fighting a boss and would like a few turns to heal/run away. Third, even if it doesn't kill a dangerous enemy outright, it removes it from the battle for four global turns (seven of your turns) which is usually plenty of time to take care of its dangerous friends. It is for this reason that I recommend getting the Rune of the Rift before level 24. And fourth, enemies that are sent into the future come back without their sustains. This makes the Rune a great opening salvo against bosses with bone shields, etc, but it also has great utility against corrupters: it is one of the very few tools you have for escaping the Fearscape, when a randboss with 34,584 HP and 89% resist all sends you there and is dealing you 200 unresistable fire damage per turn.
Wild infusion is self-explanatory. There are several "secret ToME tests" like this: noobs that fail doom their characters to inevitable death, while players with enough experience pass and have a good chance of winning the game. One of the first such tests is whether you keep your starting wild infusion, or swap it out for something worse (a.k.a. anything else). See the "what will kill you" section below for some reasons why.
The healing runes are more subjective. I like to keep two healing rune on hand, under the theory that you can *never* have too many non-spell healing talents on hand. Controlled phase door, teleport, movement, more wild infusions... many other choices are also fine. (Notice, though, that movement infusions are rather silly for archmages, and for Essence of Speed archmages in particular!)
Notice that there is no manasurge rune here. You do want to keep a manasurge rune until at least mid-game, when Disruption Shield can be used instead, freeing the slot.
Prodigies
You have several good options:
1. Cauterize. A great survivability boost for any character. On one with less than a thousand HP? A no-brainer. Do whatever it takes to unlock this; stand in your own Inferno if you have to.
2. Meteoric Crash. This is my second choice -- it adds great DPS and crowd control to your arsenal, stunning enemies is always useful, and best of all, it's completely passive and free. Don't get this before you've finished all escort quests, though. Trust me on this.
3. Corrupted Shell. Another good choice. A Yeek is probably increasing his max HP by 25% with this prodigy.
4. Spine of the World/Unbreakable Will/Draconic Will: Good, if rather boring, protection against debilitating status effects. With Cleansing Flames and Relentless Pursuit, though, you won't really need them.
Class Talents
Spell/Arcane
1. Arcane Power (recommended talent level: 5, ASAP)
A straight-up boost to spellpower. This very noticeably improves the DPS of your spells in the early game -- always a good thing in the Ritch lair/halfling ruins, which is why I recommend investing the full five points ASAP. In the mid-game, you can turn this off to conserve mana under Essence of Speed, and likely won't even notice due to the diminishing returns of spellpower. Turn it on again late-game when you have nothing better to do with your mana.
2. Manathrust (5)
The best of the tri-beam spells, since almost nothing has arcane resistance. Spam this all game long.
3. Arcane Vortex (1)
Complete garbage, but you need this to unlock...
4. Disruption Shield (1 ASAP)
By far the best shield in the game, and, sadly, you can't take full advantage of it, since you won't have enough of a mana pool to keep this active reliably. Still, the Disruption Shield is an irreplaceable talent for an archmage. Turn it on when low on mana, and it serves as a manasurge rune that also makes you invincible for several turns! Unless extremely low on health, there's no need to get paranoid about letting enemies explode the shield -- with the size of your mana pool, the damage is negligible. Don't forget you can also manually cancel the shield if necessary.
Spell/Flame
1. Flame (5)
What can I say? One of the classic beam spells. It becomes your best spell to spam once you've invested in the Wildfire talents.
2. Flameshock (1 ASAP, 5 eventually)
Stunning enemies is always a good idea: they do less damage to you, and can't hit you again with their most annoying active talents. Unfortunately, unlike Freeze, Flameshock is all-or-nothing: either it stuns and burns enemies, or it does jack all. It's still great for crowd control, or for dealing with physical enemies that come too close for comfort, and of course late game it's yet another way of spraying your Burning Wake everywhere.
3. Fireflash (5)
Your best crowd-control spell, and cheap, too. Unfortunately, the projectile is quite slow, and feels even slower with Essence of Speed on -- there's little point firing off a Fireflash when your tribeam will kill the enemy before it even arrives. At maximum talent level, Wildfire gives you 90% resistance(!) to splash damage from your own Fireflash, so there is not reason not to detonate a fireball at point-blank range if doing so will kill the enemy more quickly.
4. Inferno (5)
The DPS is not impressive, but it lasts practically forever, so gives you a good way of "fumigating" vaults: launch a Fireflash, follow up with Inferno, then teleport away. Come back after resting to mop up any survivors.
Spell/Wildfire
1. Blastwave (1)
Knocks back enemies that get too close. Given that the damage is unimpressive, that it only does anything to enemies near you (which they shouldn't be in the first place), and that the enemies you most want to knock away have high knockback resistance and physical saves, there's no reason to sink more than one point here.
2. Burning Wake (5)
Not a critically important spell, but it adds damage to all of your fire spells, so it's a fine place to put your talent points late-game.
3. Cleansing Flames (5)
Removes your debilitating status effects, and your opponents' beneficial ones? As a free passive? A no-brainer.
4. Wildfire (5)
Yes, another sustain, but very much worth it for the insane fire resistance and damage penetration bonuses. Run into a pack of fire-immune fire wyrms? Who cares, launch a Fireflash and cackle with glee.
Spell/Air
1. Lightning (5)
The weakest of the tri-beam spells, it's also the only one that penetrates enemies at talent level 1, which is critical for clearing ritch pits in the second starting dungeon. Unimpressive past the very early game, but you need *something* to do with all of your extra turns, and Lightning is quick, cheap damage.
Spell/Water
1. Glacial Vapour (1)
A very weak DoT spell. You are here only for...
2. Freeze (1 ASAP)
Freeze (the status) is one of the best status effects in the game, and Freeze (the spell) lets you dole it out often and for cheap. In the early game, Freezing all archers, mages, corrupters, bosses, etc. ASAP should be your MO. They are completely harmless until they manage to break out of the ice, they will waste all of their best active talents to do so, and in the meantime you can soften them up with your own damage spells. Bosses at high levels are increasingly likely to save vs. freezing (but unlike Flameshock, the spells still deals damage when it fails to freeze.) Still, the utility of this spell cannot be overstated: even in the late game, there is often some enemy (orc wyrmics/corrupters, horrors, grand master assassins creeping up from behind, etc) that you really don't want to deal with while you take care of a more pressing problem.
You only want to invest one point, though. At higher talent levels, Freeze deals more damage, but also has higher cooldown. You're here for the debilitation -- the damage is only a bonus -- so, strangely, Freeze is one of the few talents that is actually worse the more talent points you sink into it.
Spell/Temporal
1. Congeal Time (1)
This looks pretty good on paper. Slow is one of the best status effects in the game, and the Congeal Time projectile can pass through (and debilitate) multiple enemies. In practice, though, it's fairly terrible. The projectile speed is slow: so slow that, with Essence of Speed, you are likely to kill the enemy before it gets hit -- and if you don't, odds are good that the enemy has accidentally sidestepped out of the projectile's path. You can always fire at an enemy standing next to you, I suppose... but if it's dangerous enough to be worth slowing, you probably don't want to waste time standing right next to it in the first place.
2. Time Shield (5)
Time Shield was good back when it dealt damage to you after use -- and then they buffed it! There is no reason not to erect it before every significant fight, before you use Aegis, to gain ~1000 extra effective HP. Buy it, use it, love it.
3. Time Prison (1)
Removing an enemy/escort/Melinda from the game for many turns has plenty of uses... but at 100 mana, the price is ludicrous. You can get many of the benefits (at least against enemies) with the completely free Rune of the Rift.
4. Essence of Speed (5)
Build-defining. As I said above, there's no point activating this talent until you can pay its huge sustain cost and still spam your damage spells with the remaining mana... aim for a mana pool of size about 500 to start with.
Generic Points
Race/Yeek
1. Dominant Will (1)
Worst case, the victim stops attacking you and then commits suicide in several turns. This is already excellent for an instant, free talent! If you're lucky, the victim will do some moderate damage to other enemies, and soak up some dangerous enemy talents, before it buys the farm.
Pay attention to the talent restrictions -- in particular, it does not work on undead or bosses. It also has a chance to fail vs. living enemies, but failure does not cost you any game turns, so there is no reason not to try to "charm" the most dangerous enemy in range at every opportunity. In fact, Dominant Will is the best way of dealing with many of the game's most annoying and dangerous enemies: luminous horrors, oozing horrors, elven cultists, oozemancers, mage hunters, orc spellcasters... Dominant Will regulates all of them. Lastly, it is possible to "stair-scum" with Dominant Will: enter a new level, charm any enemy within range, then pop back up the stairs. Since Dominant Will is instant-use, this lets you kill nearby enemies at no risk.
Higher talent levels decrease the cooldown, but since battles typically will not last 30+ turns (and those that do, such as the duel with Atamathon, do not typically involve bystanders) there is little reason to do so.
2. Unity (5)
The second of the Yeek's four excellent racial talents. +60% immunity to two of the most deadly status effects for magic users? Yes please. The bonus to mental saves is the cherry on top.
3. Quickened (5)
Self-explanatory.
4. Wayist (1)
The weakest of the racial talents is still well worth the 1 talent point. Don't expect the mindslayers to do much in and of themselves, but every bit of damage they soak up is damage that you don't have to heal from. They also serve as great distractions when you are cornered and can't escape, such as the during the various ambushes, or when fighting tough bosses like the Master, Atamathon, and the sorcerers.
Technique/Combat training
1. Thick Skin (5)
No-brainer. There isn't a single character in the game that couldn't use more +resist all.
2. Armour Training (1)
The last thing you need, with your tight mana pool, is a lot of gear with +15% fatigue. But odds are good you will eventually run into some nice gloves, boots, or even body armour that will need armor training to wear, so might as well set aside the point you need for it.
Spell/Conveyance
1. Phase Door (4 ASAP)
Along with teleport, one of your two mobility talents. At level 4, you have controlled phase door of either yourself or an enemy/ally. This allows you to escape, reposition yourself behind cover, reposition an escort behind cover, etc. Keep in mind that if you try to Phase Door outside your line of sight, there is a chance of failure, in which case your destination gets reassigned randomly -- do not rely on a phase door through walls as your only means of escape (if you have at least one turn's worth of health as a buffer, though, attempting a phase door into the fog of war, followed by some other means of escape (e.g. teleport) if things go horribly wrong, is a perfectly good tactic and works out well more often than not.)
The last talent point increases the talent's range by one square. A perfectly fine place to put an end-game generic point, but not needed in the early game.
2. Teleport (1 ASAP)
Your other mobility talent, for more severe emergencies. Get one point as soon as possible; controlled teleport is a nice luxury (and eases the pain of the Sandworm dungeon considerably), but not essential in the early game.
Keep in mind that there are time when Phase Door will function but Teleport will not, and vice verse. For instance, Teleport won't work on certain small maps (such as the orc ambush following Dreadfell) and Phase Door won't work inside vaults. Worse, if the talents fail, they still take up a turn, and this is a classic way to YASD. Be careful.
Neither work in the Charred Scar (well... they won't fail outright, but you won't be happy with the results.)
3. Displacement Shield (5)
The Rune of Reflection's ugly step-cousin. It's annoying to use for several reason: it requires you to pick one target enemy (instead of always reflecting back on the attack's source), has a maximum range (albeit a generous one), takes a turn to cast, only has a chance of blocking attacks (and it is easy to get confused about how many points of displacement shield vs. "real" shield you have left, with deadly consequences), and takes an eon to cool down. Still, Displacement Shield stacks with all of your other shield, and redirecting some extra damage towards bosses always helps, so this is not a bad place to put extra generic talent points.
Spell/Aegis
1. Arcane Reconstruction (1 ASAP)
Heals you. For quite a lot of HP, actually -- more than any other talent or rune, once you have decent spellpower -- and with Arcane Shield, this translates into big damage shields. You'll only need 1-2 talent points here for Arcane Reconstruction to fully heal your HP, but since the healing (and shielding) amount scales quite well with talent level, you might as well invest the full five points by the end of the game.
2. Shielding (5 ASAP)
3. Arcane Shielding (5 ASAP)
Essential defenses; max these ASAP and keep them on at all times.
4. Aegis (1)
This talent will almost double your shield's strength at high spellpower, and does so at instant speed, so you definitely want to pick this up. Unlike Arcane Reconstruction, the bonus scales very poorly with talent level, though, so there is not much incentive to invest more than the one point.
What Will Kill You
1. Burst Damage (danger level: **)
In the early game, this includes attacks from nearly anything: archers, casters, physical enemies with Rush, assassins, etc. Surviving burst damage is mostly a matter of awareness and common sense: activate your best shield rune as soon as something dangerous comes into line of sight, and debilitate (Freeze/Stun) the threat before it can unload its best talents at you. Phase Door/Teleport away at the first sign you are losing control of the situation.
By late game, only a few enemies will remain that are capable of significantly denting your massive shield defenses. Elites and bosses always command respect, as do certain specific enemies you will soon learn and recognize; some of the worst offenders are listed separately below.
2. Damage over time (**)
There are tons of talents that do various types of damage over time: diseases, acid clouds, burning, poisons, etc. Few of these are any real threat, but sometimes powerful casters can inflict DoT status effects that deal a surprising amount of damage per turn -- burning in particular seems to be prone to dealing heavy damage. Certain DoT status effects also reduce the effectiveness of healing, which can kill you if you're not paying attention. Impending Doom is the most notorious, if rare, example of this, but look out also for Insidious Poison. None of these talents impede or prevent you from using shields, though: put on a shield, teleport to safety, and wait out the DoT.
3. Stunning (**)
While stunned, your spells deal less damage, and worse, do not cool down. This becomes a fatal problem when all of your escape and damage spells are on cooldown, and is the reason that a wild infusion is essential throughout the entire game. Fortunately, enemies in the early game are only capable of stunning you for 3-5 turns; in the late game, it is possible to get stunned for dozens of turns, but by then you should have stun immunity gear, Relentless Pursuit, and Cleansing Flames to help deal with the problem.
4. Freezing (*)
The screen turns green and you are unable to move, teleport, or heal. If you have plenty of health and shields left, you might as well wait it out... enemies will deal reduced damage to you while you are in an ice block, and help break you out. Healing talents will not work while you are trapped, but shields work just fine (this is another reason to keep around one shielding rune instead of switching to pure healing). Your second option is to proactively break the ice with your most powerful spells. Only a few bosses (notably, the mummy lord in the Elven Ruins) are capable of freezing you inside a significantly durable ice block. Finally, you can use a Wild infusion/Relentless Pursuit to remove frozen status immediately (i.e., if you have low health, and all of your shields are on cooldown.)
5. Silence (***)
You can't cast spells (including your runes). Obviously, this is a problem, and one that must be remedied (with e.g. your Wild infusion) ASAP. Fortunately, enemies that silence are very rare, and silence immunity is easy to acquire (you get +60% immunity from passive talents, for one).
6. Confusion (***)
Your actions have a high (typically around 50%) chance of failing and still taking up a turn. As an exception, Wild infusions never fail, fortunately. If you are in no immediate danger, you can just fight through it... carefully, since failing to heal or teleport 3+ times in a row is not uncommon. For Yeeks confusion is less of a threat than for other races, thanks to the passive confusion immunity bonus and mental save bonus.
7. Blind (****)
You can still cast all of the spells you want, but can't see anything. An extremely frustrating status effect. Your only option, if you can't cure it, is to teleport away and wait it out -- unfortunately, long-lasting Blind is quite common in the late game, and I strongly recommend getting equipment offering high levels of blindness immunity. Notable bosses with devastating blindness attacks are the Sandworm Queen and the overpowered multi-hued wyrms in the Vor Armory. The latter love to spam sand breath that can blind you for 40+ turns -- more than Relentless Pursuit can cure. Do not unleash the wyrms without a reliable plan against blindness!
8. Spell Disruption (*****)
Like Confusion, but only affects your spells/runes, tends to last forever (dozens of turns), and worst of all, is easy to miss if you're not paying attention: unlike Confusion or Silence, there is no obvious visual cue that you've been hit by Spell Disruption (other than the icon on the right of the screen, of course). You get Spell Disruption from attacking certain anti-magic enemies with spells: Champions of Urh'Rok are the most common and, fortunately, have a very distinctive tile; notable bosses with Spell Disruption include Gorbat and Protector Mairsil.
Spell Disruption has a cooldown, so once you cure it with your Wild infusion/Relentless Pursuit, you can safely spam your best spells.
9. Slow (extreme!!)
Back when ToME 3.0 was still in beta, and the Master was the final boss, stun used to skip your turns instead of just disabling your talents: you would be playing when all of a sudden, your controls froze and the game began to lag like mad. Particles flew everywhere, and 30 seconds later, dun-dun-DUN!, you are back in control, have -4020 health, and you would have better luck reassembling a Ming vase at ground zero of a nuclear bomb than piecing together exactly what happened from the game log.
At some point people realized that going from full health to -4020 without any control whatsoever was... not terribly fun... and stun got toned down. Freeze and Daze eventually got similar treatment, leaving only one status effect that can completely screw a character with full health and thousands of virtual shield hitpoints: very high amounts of Slow.
Ludicrous levels of Slow seem to occur when enemy Chronomancers trigger anomalies, and there is some indication that current behavior is a bug. Until it is sorted out, treat all elites with high paradox as an extreme threat. Once you're slowed, there's nothing you can do... there is some poetry to that. We live by global speed, and die by global speed.
10. Sustain disabling talents (e.g. Disperse Magic) (**)
There are two sustains you don't want getting shut down in the middle of a tense fight: Arcane Shield and Essence of Speed. Losing the others is not typically fatal (although watch out for Fireflash splash damage if Wildfire gets disabled). These talents are more of an irritation than a direct threat -- the danger comes from missing the fact that they have been disabled and making play mistakes.
11. Mana Clash (***)
At low levels, Mana Clash is a minor annoyance, stripping away some of your MP. At high levels, Mana Clash takes away all of your mana, shutting off your sustains and cutting off your escape options. This is the time to activate Manasurge/Disruption Shield and then teleport away at the earliest opportunity. Mana Clash is only truly dangerous when fighting enemies that can activate it several times in succession, stripping you of your mana and your backup talents for generating mana.
12. Corrupters (***)
Powerful casters at all points of the game, full of ranged nukes and powerful AoE -- Freeze these guys on sight. The Fearscape can prove a deadly surprise, especially late game where the flames of hell deal hundreds of irresistible damage per turn and the Corrupter in question has 40,000 max HP and is healing the same. This is the time to use Rune of the Rift.
When cast by weaker enemies, though, Fearscape can help you more than it hurts you, separating the annoying Corrupter from his friends until you're done killing him.
13. Oozemancers (****)
For some reason, every time the developers add a new class to the game, they make that class 100x more powerful than anything there before. When people complain, the new class gets toned down a bit from "ludicrously OP" to only "highly OP."
And in the case of Oozemancers, this class is specifically designed to counter your character. Lovely.
Oozemancers used to one-hit kill players from outside line of sight, and/or heal for thousands of HP per turn, on a regular basis. Fortunately that doesn't happen anymore, but Oozemancers have tons of anti-magic talents and command considerable respect, especially in tight quarters where you can't easily escape, such as during ambushes. Under no circumstances should you enter or remain inside their auras' areas of effect!!
14. Luminous/Radiant Horrors (*)
All horrors should be treated as potentially dangerous until you're familiar with them, but the Luminous Horror and his big brother are particularly notorious, due to their healing talents, burst damage capabilities, fire immunity, and tendency to come in groups of three. They don't pose a real danger to Yeek archmages, though.
First, they are susceptible to Dominant Will. Charm one of them, teleport away, repeat (x2). If for some reason you need to kill them the old-fashioned way, use Manathrust, Lightning, and Rune of the Rift to dish out damage, and Freeze to stop them from healing.
15. Umbral Horror (***)
A particularly annoying horror, this guy hides inside a cloud of darkness (so you can't target him) and sends shadows after you. The shadows hit like a truck, have an annoying tendency to appear while you're busy fighting another dangerous enemy on the other side of the map, and are immune to all damage for several turns after they appear.
Wading into the darkness to try to find him is an exercise in frustration; instead, Phase Door away and wait for him to come to you, or kite him into a narrow corridor where your spells can't miss.
16. Worm that Walks (****)
Another notorious horror. In the past, I've had Worms that Walk hit me with ranged spell that deals 300 blight damage per turn, for a dozen turns. At level 30. This hasn't happened in several game versions, so maybe it was a bug... in any case, it can't hurt to treat him with caution. Also do not, under any circumstance, attempt to engage him at melee range.
17. Oozing Horror (extreme!!)
Worst of a bad lot, the Oozing Horror is a real nightmare for an archmage. His worst attack teleports you next to him, pins you, silences you, and drains all of your mana. In one turn! Hit him with Dominant Will and thank your stars you're playing a Yeek! In a pinch, Rune of the Rift can also be used to break free and, hopefully, escape.
If you run into a randboss Oozing Horror... that may be the one time it might be prudent, as a Yeek archmage, to permanently flee the area.
Who Will Kill You -- Notable Bosses
1. Subject Z (*****)
First, note that there is no requirement to kill Subject Z the first time you enter the halfling compound. If you do decide to engage him, prepare for the most frustrating fight in the entire game. Unless you've been lucky and found a nice shielding rune or +HP equipment in the first two dungeons, Subject Z has a good chance (about 50%, I'd say) of one-hit killing you should his Shadowstep connect. So... don't let him do that. He can Phase Door, so a large distance between the two of you is no guarantee of safety.
If you're trying to save the Yeek (and if you're fighting this battle early, presumably it is for RP reasons), keep in mind that you should have targeted Phase Door by now, and can use it to teleport the Yeek to safety behind you.
2. the Master (***)
The Master's most dangerous talent is his ability to summon undead all around you. If this happens, teleport away: the Master will come find you, but his minions will not (and if they get too far from you, they will unsummon automatically). He also can Freeze and turn invisible, which can prove annoying, but overall with Essence of Speed and plenty of shielding, this should not be a difficult fight.
3. Orc Ambush (*)
This is the first fight that's more about attrition than short volleys of spells: you will need to carefully manage your mana and talent cooldowns so that you always have a way of healing, escaping, and restoring mana as needed. Not at all a difficult encounter to win if you play carefully, though, and good practice for the final boss.
4. Mark of the Spellblaze (****)
One of two western dungeons that I recommend saving for after Dreadfell, due to two tough back-to-back fights against the Grand Corrupter and [strike]the Imperial Wizard[/strike]Protector Mairsil. The Corrupter will send you to the Fearscape, and Mairsil has Spell Disruption. Moreover the Anti-antimagic quests is a bit buggy, and it is possible for the Grand Corrupter to turn hostile during/after you have killed the enemies in Zigur; since he is invincible at that point in the quest, the encounter can prove... challenging.
5. Celia (*****)
The other dungeon that I recommend saving until after Dreadfell. For some reason, the undead in Celia's crypt are way overleveled compared to the rest of the content in the west, making the Celia crypt the second-biggest noob deathtrap after the Melinda crypt (and only because to find Celia, you have to poke around in town, vs. Melinda tempting you directly with a pop-up). Once you've dealt with the undead in the coffins, Celia herself is no slouch, and possesses extremely hard-hitting burst damage spells. Keep a shield up at all times, and hit in the side crypts to heal if necessary.
6. Vor Amory Wyrms (***)
Not that tough if you exercise a bit of common sense. First, their most dangerous attack is sand breath, which can hit you from out of line of sight and can blind you for 40+ turns. Do not engage the Wyrms if you have no reliable way of dealing with blindness! Second, tanking more than two of the wyrms at once is suicidal, even with good shielding runes. Kite them one at a time. Fortunately, the wyrms have a tendency to stay in their vault until provoked (so don't wander inside until most are dead, obviously!). Lastly, carefully plan your escape strategy, since you will likely need to heal between kills -- in particular, look out for teleport traps in rightmost room of the armory, which can be used to your advantage.
7. Pride Bosses (**)
All of the Orc bosses are somewhat dangerous and should be fought carefully. Gorbat in particular has anti-magic talents, including Spell Disruption, and a tendency to turn off your sustains. Vor has Meteoric Crash, which hurts if you're not prepared (and is also a way of unlocking the talent for yourself, if it hasn't happened yet at random in a dungeon).
8. Atamathon (*****)
The "real" final boss of the game. Since for you the High Peak is a one-way trip, Atamathon must be fought beforehand. He has very damaging attacks and can Rush and Arcane Pull, so it is critical to keep up a damage shield (at least 2000 HP's worth recommended) at all times he is in line of sight. He has high HP but no way to heal, so the name of the game is attrition: hit him until you are running low on shielding, then teleport away (possible using your Wayist friends as distractions to cover your escape) and rest.
9. The Sorcerers (***)
If you can take on Atamathon, the sorcerers are not much of a challenge. Let Aeryn tank their hits while you turn off the portals, then focus on the sorcerers one at a time. If things go badly and you need to lose line of sight to one or both sorcerers, there are conveniently placed pillars around the room, and Phase Door and Teleport both work on this map.
Gear
With the new artifact system, you can no longer find any particular artifact with high reliability on every playthrough. Keep an eye out for the "classically good" archmage artifacts, like the Lost Staff of Archmage Terelion, but also look over all orange randarts you find, since if you're lucky they can have some truly sick stats.
Modifiers you are looking for in particular include:
1. +life (also, "only die when reaching")
You want extra life almost to the exclusion of everything else in the early and mid-game. +CON also helps, to a lesser extent.
2. Healing mod
In the late game, healing mod becomes the most important stat, instead of flat +life. The reason is that most of your protection comes from shield, which come from healing effects... which scale up with healing mod. +100% healing mod effectively doubles your HP.
3. Stun/Blindness immunity
The two status effects you don't have any passive protection against; the more immunity, the better.
4. Saves
Shrugging off annoying status effects is great, and your saves can never be too high.
5. Spellpower
Small amounts of spellpower don't really matter, particularly starting in the mid-game when you have a ton of spellpower from MAG, but when an item provides spellpower in huge chunks (e.g. +20) that is well worth considering.
6. Spell crit chance
As usual, for this build I consider survivability more important than raw DPS. But once you have the above bases covered, having good crit chance makes the game all that much easier.
Miscellaneous
Escort Rewards. The one you are really looking for is the Anorithil's Healing Light: a free healing spell that stays useful all game long, and is particularly helpful in the early game when you don't have slots for healing runes. Chant of Fortitude gives a big, free bonus to your saves and is also an auto-pick. For the rest of the escorts, the passive bonuses (Heightened Sense, Piercing Sight, Unflinching Resolve) are usually much better than the active talents or stats. I recommend taking either the passive bonus or, for escorts that don't offer one, +2 MAG.
Brotherhood of Alchemists Reward. To be honest, these rewards are so weak, and the whole quests has such a large luck component, that it doesn't really matter what you take. Extra talent points and spell crit are the best rewards, I guess.
For the final reward, the Potion of Invulnerability is a lot worse than it sounds: unlike the Eidolon it doesn't heal you, restore your mana, or remove negative status effects, which are probably the reasons you are about to die in the first place. Still, it can buy you a few turns as you wait for a critical talent to cool down, or rush for the stairs, etc, and since the rest of the final rewards are so terrible, you might as well go for the potion.
Lost Knowledge. This quest used to be extremely frustrating; nowadays keeping Limmir alive is much easier, and the quest reward well worth it; the only problem is that Limmir's tome may not appear in your game at all.
If you do receive and complete the Lost Knowledge quest, the best gems to socket are Atamathon's Ruby Eye (guaranteed) and the Goedelath Rock (not guaranteed), which combine to give you very nice bonuses, including +5 to all stats, +20%/+9% fire/all damage, +16 spellpower, +60 life, and last but certainly not least, +50% heal mod.