Walk through of early game on insane (should be helpful if you are new to insane)
Part 1: https://www.youtube.com/watch?v=x9Vw3746mAc
Part 2: https://www.youtube.com/watch?v=4B5n9h1sMAw&t
Part 3: https://www.youtube.com/watch?v=v4jcnVg9QHY&t
Part 4: https://www.youtube.com/watch?v=CnsyL2Yqxyo&t
Walkthrough of Highpeak and final battle:
https://www.youtube.com/watch?v=p_xKPXHqMQY&t
TLDR Version:
Stats: Dex > Cun > Str > Will
Prodigies: anti-magic: superpower/windtouched speed, arcane: arcane might/eye of the tiger
Race: Halfing, Ogre, Krog, Doomelf
Class Talents: (pick either poisons or traps)
Buckler Training 5/1+/5/1+
Skirmisher Slings: 5/3+/3/0
Tireless Combatant: 3+/1/3+/5
Called Shots: 4+/3+/3+/5
Poisons (Locked): 2/1-2/4/0
Traps (Locked): 4+/1-2/1/4
Generics (pick 1 locked tree/escort tree 2 if cornac)
Combat Training: 5/1/3/3+/0/0/0
Mobility: 2/1/2/3+
Survival: 1/1+/1/1
Antimagic: 5/1+/3+/0+
Skirmisher 1-2/2+/3-4/2
Fungus: 1+/1/3+/0-1
Chants: 3+/1/4/0
Light: 1/3/1/3+
Tinker: 4/2/1/0 , 3/0/0/0
1. Why Skirmisher:
Skirmisher is one of the simpler and older classes in Tome4. The few trees it does have unlocked are quite strong, but all the locked trees are mediocre. With a small but viable core, you can build on top of the class however you want. Anti-Magic with fungus and traps? Sure! Undead with scoundrel and poison? Sure!
The class excels at attacking and disabling enemies from range but struggles with damage during the mid-late game. Slings do about as much damage as a 1h strength weapon assuming prodigies. However, their base attack speed is 125%. This means it takes 80% of a turn to attack. With windtouched speed you get 120% of a turn each round, meaning you act 1.5 times to each time an enemy does. With Nithan's force a 143% speed sling it takes 70% of a turn to act. With a combat speed amulet and aclarity chest that can get down to 56% of a turn to attack. This allows you to stack debuffs with poison and scoundrel (if you go that path) very quickly. The passive regen from tireless combatant provides enough sustain to keep you going without escort trees. While not strong, the core set is adequate and allows you to take a combination of locked trees that suits your style of play.
Pros:
• Simple
• High Mobility
• Flexible (requires no cat points and few class/generics to function)
• Locks down enemies with disables.
• High Sustain
Cons:
• No way to deal with statuses
• Low damage
• No standout prodigies or locked trees. The class spams the same 4 abilities and plays exactly the same at level 15 as it does in high peak.
• Low Burst
2. Stat Points:
Early: Dex > Cun > Will > Str for unlocks.
Late: Dex > Str/Will/Magic (Prodigy Dependent) > Cun > either Str/Will
See prodigy section for further details.
3. Race:
The class requires very few generics or class points. You will want to either pick a race with good places to dump talent points or go anti-magic. I highly value life rating as each point is a greater ego on gear. I'll rate them out of 5. Keep in mind, even though ghoul/skeleton is a bad choice for skirmisher because undead, skirmisher might be a solid choice for ghoul. You can really play any race you want here. (I'd like to rate yeek -1/5 but there'd be riots)
• 2/5 Cornac (10): Would need to go anti-magic and unlock two generic trees to make use of generic points. Would get no benefit from extra class points.
• 2/5 Higher (11): Resources are not an issue for skirmisher nor do they care about spell power. At least offers blindness immunity and an instant regen + heal mod.
• 3/5 Thalore (11): Helps with resall stacking. Would work with an anti-magic build.
• 3.5/5 Shalore (9): +Global Speed!, The only beneficial effects you'd care about are evasion and PES. Could work with a PES + AM build. More crits and crit mod is always nice.
• 3.5/5 Doomelf (9): The teleport + resall. Pitiless on stuns/pins/debuffs. Crit shrug on a dex stat class. All good! 345 less hp at 50 than ogre/krog.
• 4.0/5 Hafling (12): High synergy. The first talent buffs crit rate and saves for 5 turns. The final talent removes stuns/daze/pins and grants immunity.
• 2/5 Dwarf (12): Saves and armor. The wall walking seems situational too. One of the weaker choices.
• 4/5 Drem (12): Frenzy allows you to skip eye of the tiger and take AM + PES. You shouldn't be in melee for black blood. Confusion resistance + mental saves is nice. The last talent is busted strong.
• 3.0/5 Yeek (7): The HP rating is painful, but global speed, confusion res, and summons are nice. 520 less hp at 50 than ogre/krog.
• 4/5 Ogre (13): You don't care about orgewield, but you do care about infusions. Seems good!
• 4/5 Krog (13): Stuns + kneecapper are busted and krog stun is the most busted stun. Also grants you around 20% phys resist, stun res, infusion cd reset on big hits, and the last ability gives resall.
• 1/5 Ghoul (14): Your base speed is 80% and your attacks take 80% of a turn so it cancels out. You have the generics to dump. Still undead.
• 1.5/5 Skeleton (12): You have the generics to dump. Lack of movement/heroisms/wilds makes it bad.
4. Class Talents:

Buckler Training: Early: 1/1/0/0, Late: 5/1+/5/1+
None of your defenses as a skirmisher are particularly strong, but they all stack! This tree provides evasion against melee and projectiles. Unfortunately, avoidance is weaker than damage reduction. In a game as long as tome4 you will have long streaks of low rolls.
Buckler Expertise: Deflects attacks in melee. It scales with talent points and cunning reaching 25%+ by endgame. Put 1 point in this early to unlock the next talent. Eventually you will want 5/5. Unfortunately this does not additively stack with evasion. Endgame with 28% from this talent and 38% from evasion enemies in melee had a 44% chance to hit me.
Bash and Smash: Knock back an enemy with your shield and then perform a sling attack. This requires ammo to use. The knockback hits 2 spaces at 3 talent points but otherwise it does not scale well. It does great damage but you won't regularly be in melee. 1/5 early. 3/5 luxury.
Buckler Mastery: Deflects projectiles. At 5/5 it makes bash and smash a guaranteed critical. By mid game you'd like to have 3/5 here for 31%. Eventually you'll max this for 44%.
Counter Shot: This gives you a free hit whenever you evade an attack with mastery or expertise. 1/5 in this whenever you start putting points into those skills. Additional points increase the number of times this can trigger per turn and the damage these hits do. Useful for applying onhits, poisons, and scoundrel effects. I can't see wanting moe than 2/5 in this.

Skirmisher Slings: Early: 5/1/0/0 Late: 5/3+/3/0
Sling Supremacy: This is your weapon mastery talent. Max this as soon as possible. Also increases reload rate. This is part of the reason you have so many extra generics.
Swift Shot: Good damage with a CD of 5. Takes 40% of a turn or less to use. Is not a called shot though and travels as a projectile. 3/5 at the minimum but probably 5/5. One of the 4 activated abilities you will cycle all game.
Hurricane Shot: You fire multiple shots in a cone. At 3/5 you spend 10 shots and can hit targets up to two times. This goes from unusable to fantastic with a high capacity ammo. Decent damage, your only AoE, but more importantly 2 shots on a target helps apply your on hits. Synergy with poisons and scoundrel. 3/5 when you have points to spare and have a high capacity ammo. One of the last places you should be spending points.
Bombardment: For the low cost of 25 stamina a shot, you now fire 3 attacks at a time with your basic shot. 25 stamina per shot means 75 stamina each attack. Could be good at 25. Literally unplayable even with the massive amount of stamina regen this class gets.

Tireless Combatant: Early: 3/1/3/0 Late: 3+/1/3+/5
This tree is surprisingly good. Running around the T1s with 30 regen makes life easy and endgame it's all the healing you need. This is the second tree entirely dedicated to defense. You will also never run out of stamina.
Breathing Room: This provides stamina regen when not adjacent to an enemy. At 3/5 it provides health regen as well. Increased by 20% by the final talent in the tree. 3/5 this at a minimum. At 5/5 with the 20% boost it reaches 8.9 stamina/hp a turn.
Pace Yourself: Reduces fatigue at the cost of global speed. Awful. Never use this.
Dauntless Challenger: This provides regen for each enemy in sight up to 4. Again at 3/5 it provides health as well. At 5/5 with 20% it provides 7.4 hp regen per enemy. 3/5 this in the T1s and run around with 30 regen all the time and never run out of stamina. Life starts to get a lot easier for you.
The Eternal Warrior: This provides stacking resall each time you use an ability that costs stamina. The duration is short but resets each time you use an ability. At 5/5 it increases your other abilities in the tree by 20%. 5/5 this. With 5/5 thick skin you get up to 30% resall from talents. With just a wild you are within spitting distance of the rescap. Res stacking is one of the core pieces of this classes defenses.

Called Shots: Early: 1/1/1/3 Late:4+/3+/3+/5 but probably 5/5/5/5
The first three abilities are your bread and butter. Stun + Slow Movement is your gimmick. You are going to ride that train to end game. All three attacks travel instantly and pass over enemies in the way. The final ability lowers their CD to 6 and provides crit, +crit damage, and 50% respen on these attacks. You will spam these abilities on cooldown and take either windtouched speed or eye of the tiger as prodigies so you can spam them more often.
Kneecapper: Pins for 1 turn and applies slow movement for 3. Also does reasonable damage. Breakpoints for slow movement are 3 and 5. The pin goes to 2 turns at 4 points. Slow movement is additive with stun. If you reduce an enemies movement speed to 10%, it will take them 10 turns to move 1 space. This absolutely destroys enemies who are not stun immune and don't have movement speed buffs. It won't work against everything, but when it does work enemies will just appear to do nothing for several turns.
Kill Shot: This gets bonus damage added to it based on range. The bonus damage goes from -50 to 250 and scales non-linearly like most things in tome4. It's beneficial to use this at even a few spaces away. Additional points only add to the base damage so this is the lowest priority for points. It also costs a whopping 35 stamina to use.
Noggin Knocker: You fire three shots. Three chances to apply onhits and proc damage. Each hit will either stun for 2 rounds, or if already stunned increase the duration by 1. Additional points will add damage. Fire this off right after kneeknocker.
Sling Sniper: Increases your crit chance, crit damage, and physical respen with your called shots. At 3/5 you lower the cooldowns of your called shots to 6. Rush 3/5 as soon as possible. Resistance penetration is extremely important in this game and this is your only source of it.

Poisons (Locked): 2/1/4/0
You'll either want to take poisons or trap mastery. Poisons would be the clear winner if venomous strike applied it's on hits with slings. It doesn't and that skill is the best part of the tree. Still, 20% talent fail and 20% less damage is pretty solid. You can switch the poisons at instant speed if you need to shut down healing. Works well with scoundrel tree to shut down enemies.
Apply Poison: at 2/5 you have a 34% chance to apply the poison for 5 turns. With swift shot and noggin knocker and high attack speed you can apply this consistently in 1 or 2 combat rounds. Additional points increase chance to apply poisons, damage, and duration. None of which you care about.
Toxic Death: On death enemies spread poisons in an AoE. At 2 points the AoE goes to 2. 1/5 is good. Never put more than 2 in this. You want to apply poison to everything? Use hurricane shot.
Vile Poisons: Each point increases the efficacy of poisons and unlocks a new poison. The first 3 are solid. At 4/5, numbing will reduce damage by 20%, insidious reducing healing received by 70%, and crippling gives 20% chance to fail talents.
Venomous Strike: Could be good, so good, but is bugged. Don't take. Please fix? If it worked I'd go 4/5 in this for a 30% global slow and to put 4 talents on CD of 6. I'd spam this all day long. It would fit perfectly with scoundrel and complete this class.

Trapping (locked): 4/1/1/4
I've never won with trapping. I don't understand the appeal of this tree. Some of the standout traps are bear trap (pins), disarming trap (disarms), anti-magic trap (resource drain + magical effect removal in AoE). People claim this tree is amazing. You want to lure enemies around corners all game long for massive disables/damage? This is the tree for you. I'd rather just shoot stuff.
Trap Mastery: You select 1-3 traps to prepare. When you prepare a trap it shows up on your ability bar. Additional points allow you to prepare more traps and increases their effectiveness. when you use a trap, it takes a turn after deployment to be ready. Then it will trigger when an enemy steps on it. The standout traps are bear trap (tier 1) that pins/slow movements, disarming trap (tier 1), poison gas (tier 2) 25% of crippling, numbing, insidious effects and good damage in AoE, anti-magic trap (tier 4) this is for anti-magic users only it dispells magical effects and does resource burn in AoE, and nightshade (tier 4) stuns. 4/5 seems to be as much as you need.
Lure: Places a lure that draws aggro. Range increases with points. 1/5? I don't have experience here.
Advanced Trap Deployment: You can place traps a space away from you. Additional points decreases the time it takes to deploy traps. 1/5.
Trap Priming: This ability lets you drop a prime trapped that can instantly trigger. This makes your traps go off when you place them under an enemies feet. It doesn't work on all traps, but it works. About half the traps can be primed. Of the ones I listed anti-magic trap and poison trap can be primed.
5. Generic Talents:

Combat Training: 5/1/3/3+/0/0/0
Thick Skin: 5/5 Stacking resall is important with this class.
Heavy Armor: 1/5
Light Armor: 3/5 Never put more than 3/5 in light armor. You don't need more hardiness.
Combat Accuracy: 3+/5. All of your on hits check accuracy.

Mobility: Early: 1/1/1/3 Late: 2/1/2-4/3+
This might be my favorite defensive tree in the game. There is so much defense and mobility in this tree. It makes pretty much anything work and carries the class. Never be in melee, ever! This is also why you will wear light armor.
Disengage: You jump back from an enemy and gain movement speed for 3 turns. At 2 points you can jump back 5 spaces and gain 160% movement for 3 turns. You have to target an enemy first, then you move in the opposite direction. It will take some positioning to use correctly. Tumble makes it easy.
Evasion: For 4 turns you have a % chance to avoid attacks. It scales well with dex but poorly with talent points. Additional points will also decrease cooldown. 1/5. Late game it gets to 35%+ chance of evasion.
Tumble: Instantly travel to a location. Breakpoints are 2/5 and 4/5. The cooldown is low and gets lower with points. Get 2/5 as soon as you can. 4/5 is a luxury. Spam this as much as you want, stamina is no concern to you. Requires light armor.
Trained Reactions: If a hit would do more than x% of your life, reduce the damage by y%. The reduction scales based on your defense. A few points in light armor mastery will get it to decent levels. 3/5 is probably enough. More points is a luxury. By endgame any hit that deals 15% of your life will be reduced by about 1/3. It'd be hard to overstate just how good this ability, you should have 3 points in this by the end of the T1s. Make sure to never hit 0 stamina or it will not trigger. This also happens before anti-magic shield. The combination of the two makes casters extremely easy to deal with in the T2s.

Survival: 1/1/1/1 or 1/5/1/1.
I won't discuss this tree. Track is probably the best ability in the game.

Scoundrel (locked): 1-2/2+/3-4/2
I enjoyed taking this tree. It combines well with poison and skirmishers high attack speed. All your disables and stuns and pins start to add up. It helps that ammo tends to have great egos too. 40% chance to apply gloom seems to be everywhere for example. You don't need an escort tree. When else would you take scoundrel?
Lacerating Strikes: Gives you a chance to apply a bleed and cost opponents a small portion of their turn. Additional points increase the chance of bleed and turn loss. I wouldn't go beyond 3/5 at the most here. 2/5 gives 25% chance. It isn't a very strong skill.
Scoundrel's Strategies: This ability is neat. You reduce an enemy's chance to crit. This effect can't be resisted. Scales 10/17/23/27/31. Enemies in high peak can have 40+ chance to crit but not always. Reducing their chance to crit you by more than half is pretty good. I'd put at least 2 points in it and more as a luxury. There is also a secondary effect. You have a small chance to cause amnesia on a bleeding target. You also only have a small chance to cause bleed. At 5 points you could get up to 25% to cause amnesia for 4 turns but.. ehh. It simply does not do much at all. Now if venomous strike worked these effects would start to add up!
Misdirection: You have a chance to shrug off physical effects. However the chance is small. I went 4/5 for 23% chance. 3/5 seems reasonable at 19%. It isn't the most impactful talent, but every little bit helps. The more effects you shrug off, the more useful your two measly wilds become. Some food for thought about saves hint hint.
Fumble: Each attack applies a 3% chance to fail a talent. This stacks up to a limit increased by talent points. At 2/5 you reach 30% cap for chance to fail. I ran a ton of simulations using various inputs for global speed and attack speed. I even included the triple attack from noggin knocker and ran millions of rounds of combat. This gives enemies around an 18% chance to fail a talent. Additional points do almost nothing. Crippling poison stacks multiplicatively causing enemies to fail around 1/3 of their talents in longer fights (think final battle).
6. Cat Points? What do?:
Most races will want to unlock 2 trees. Cornac will want to unlock 3. This is where you can build the class however you want as none of the trees is core to the build. I recommend unlocking one class tree (poison or traps) and one generic tree. You always want 5 inscription slots. Don't unlock more than 2 trees especially with this class.
(pick 1)
Poisons: Half as strong due to venomous strike bug, but still useful at range.
Traps: Strong effects do yon want to spend your turns throwing traps or moving and shooting?
(pick 1: the first 3 are escort dependent)
Tinkers: The strongest choice by far.
Light: Cleanse and +healmod/healing
Chants: Cleanse and reduce all damage greater than x distance by %? Hell yeah!
Fungus: Probably overkill with class regen but still very strong. 1/1/3+/1 is my preference for fungus.
Scoundrel: With on hits and poisons you can really shut down enemies. You don't need another defensive tree with this class.
Tinkers is always good. Light/Chants are arcane and work well with arcane might as a prodigy. Fungus is anti-magic only and allows shenanigans with fungal blood prodigy. Fungus should also be taken with superpower as it scales of mindpower.
7. Prodigies:
Skrimisher depends on spamming called shots. There are two prodigies that help you do this: eye of the tiger and windtouched speed. Pick one. Base sling damage is pretty low. You have three options to raise this: PES, superpower, AM. Each has it's niche. There is also vital shot.
Eye of the Tiger: Each time you crit you lower the cooldown of an ability by 2. This will almost allow you to use nothing but activated abilities. You should be able to do something like kneecapper -> swift shot -> nogginknocker -> killshot -> repeat. Though spaming killshot will destroy your stamina.
Windtouched Speed: 20% global speed and 10% CD reduction. Global speed gives you more of a turn to spend. Attack speed makes your actions cost less of a turn to use. This works very well together. The CD reduction floors meaning swift shot will have a CD of 4 and your called shots down to 5. A perk of going anti-magic. You will still have to equip a charm and float around 10 generics to get this.
Pain Enhancement System: When you crit you add 1/2 your strength to all your other stats for 6 turns. Strength is easy to raise on gear and this gives you a ton of +weapon damage. Problem is it's up time is bad at 33%. Strength is the easiest stat to raise.
Arcane Might: Adds 50% magic modifier to weapon. Adds spell power to physical power. Magic is harder to raise. Arcane Might is great though. You don't care about spell power, unless you took light/chants, but you turn it into physical power anyway. You just need to get magic high enough to get the buff from heroism. Do not take this if anit-magic.
Superpower: Adds 40% willpower to weapon and turns Str into mindpower. Willpower is also hard to raise, but not as hard as magic. Mindpower only maters if you take fungus. Some racials scale off will (krog/yeek) and you need to raise it to keep sustains up if anti-magic/fungus. Take this if anti-magic.
Vital shot: 450% attack modifier, 5 turn stun, and you debuff attack speed by 50%. This is great. However... if you are taking this you aren't taking a talent that increases base weapon damage or allowing you to just spam called shots more often. I don't see a 50% physical respen on this like on your called shots. Enemies end game will frequently have 70+ resall in highpeak. You can also be pulled into this attack. You are better off taking other prodigies.
Fungal Blood: If you take fungus you can gain a bunch of free turns with ancestral life. Your mindpower will be low however. Turning all your infusions into 7 free basic shots is a really bad return on investment. You want to spam called shots and get to use infusions to stay alive. Also people will be convinced that you are breaking the game or something and attack you.
PES vs. Superpower vs. Arcane Might: With numbers!
So you have three prodigies that can increase base weapon damage, which is better? I'm going to somewhat arbitrarily pick some stats and compare numbers. I'll use a 165% power ammo. One of the reasons these are so important is because heroism lasts 11-12 turns (I run 2 as should you) and provides +24 or so to your top 3 stats. These 3 prodigies above all add them them to your weapon. PES will have two numbers.
*I picked stats I thought fairly represented an attempt to build for the prodigy. As long as your 3rd highest stat is str, magic, will for the corresponding prodigy the analysis should hold
Vanilla Stats: (100 str, 130 dex, 50 willpower, 110 cunning, physical power 80)
• Relevant stats with heroism: 154 dex, 134 cunning, physical power 85
• Weapon damage: (0.3 * (85 + 0.7* 154 + 0.5 * 134) * 1.65 * 1.57 ) ^1.04 = 249
PES Stats: (120 str, 120 dex, 50 willpower, 100 cunning, physical power 84)
• Relevant stats with heroism/PES: 192 dex, 172 cunning, physical power 90
• Weapon damage: (0.3 * (90 + 0.7* 192 + 0.5 * 172) * 1.65 * 1.57 ) ^1.04 = 300
• Relevant stats with heroism: 144 dex, 124 cunning, physical power 90
• Weapon damage: (0.3 * (90 + 0.7* 144 + 0.5 * 124) * 1.65 * 1.57 ) ^1.04 = 242
Superpower Stats: (80 str, 130 dex, 85 willpower, 110 cunning, physical power 75)
• Relevant stats with heroism: 154 dex, 134 cunning, 109 willpower, physical power 75
• Weapon damage: (0.3 * (75 + 0.7* 154 + 0.5 * 134 + 0.4 * 109) * 1.65 * 1.57 ) ^1.04 = 283
Arcane Might Stats: (80 str, 130 dex, 85 magic, 100 cunning, physical power 96)
• Relevant stats with heroism: 154 dex, 124 cunning, 109 magic physical power 101
• Weapon damage: (0.3 * (101 + 0.7* 154 + 0.5 * 134 + 0.5 * 109) * 1.65 * 1.57 ) ^1.04 = 320
To be fair, I have no idea what sort of sacrifices it would take to get magic to your third highest stat. But if you do... AM is far and away the best damage prodigy. Superpower is next. PES is third. During its up time it offers the second highest weapon damage, but not by much. PES is highly overrated in general (math is hard). AM and superpower are almost universally better with the stat buff from heroism. Not mentioned during the up time of PES is the accuracy bonus, crit shrug, crit rate and talent scaling. Those are not insignificant things, but I don't know how to quantify them. In my opinion PES is something you should only take if you also have superpower/AM even on a dex/cunning class. Please don't be the guy to take PES on oozemancer over superpower. ( I did this when I was new)
8. Anti-Magic?
Make this choice at character creation. The anti-magic tree costs around 9 generics minimum and pairs well with a race that isn't generic hungry (or arcane). There are reasons for both.
My advice? Yes. Do it. Give in.
Pros: You have the generics. Krog is great. You don't lose much in terms of items as skirmisher. windtouched speed.
Cons: Time shield ego, you don't care about fungus as skirmisher, no chants/light, all the arcane only items.
I love the combination of trained reactions + AMS. Makes the maze or ruined halfing zone a joke. T2s are the hardest part of the game. AMS makes that part easier. The are several overpowered arcane items (at least one anti-magic one in guardian totem) and your stance on anti-magic probably comes down to how much your rely on these.
9. Further Thoughts
Constructive feedback is appreciated. Also, I don't see a point to discuss inscriptions or what to take from escorts. Those seem like things that are not specific to this class.
Sorry I lied about limiting the amount of math. Admit it, you didn't read the whole thing.
Edit: I think I'll add some tips/tricks
1. Some enemies will have very high deflection rates like 80%+. Deflection only works on attacks that target enemies. You can bypass deflection by targeting past an enemy.
2. You can switch pouches or use a movement inscription to very quickly reload ammo.