ToME: the Tales of Maj'Eyal

What am I supposed to do with this? Speed + Essense of Speed + Momentum + Shadow Veil. And it has Ambuscade too. It deals more damage each turn than my max hp. I only survived because of Unstoppable (after dying twice).
I can't even skip it, because then I'll have to fight with other two bosses, while this monstrosity wanders around (and next pedestal is very close).
Adventurer's class is marked "not balanced by any means" for reason - because mixing abilities from different class can produce insanity like this. But enemis do it a lot. And then it is multiplied by their skill bonuses - Shadow Veil, for example, is 9.6. I think at least time-manipulating skills should be banned for mobs. Or just their speeds capped at 100%.

You have a few options, none of the great:

- Teleport away for a short break. IIRC, Shadow Veil does not follow anymore.
- attack it with ranged attacks from range >5 (to avoid shadow veil)
- completely drain its mana/stamina to get it to lose its sustains and ability to put more up (NPC's are bad at resetting buffs). Momentum may help with this, but maybe not.
- use some other way of dispelling buffs, like gloves of Dispersion.
- Try invisibility. Even a power 1 invisibility will make you undetectable, as it has 0 see invisible.
- stack defense or armour until it doesn't damage you significantly. (tough)

Also, improve your character:
- Buy a merchant artifact, if you have saved him
- get one more level, for another prodigy.

Well, I managed to kill it without extra deaths, by strictly fighting it only while in Unstoppable and then teleporting away. And it was interesting, in a way, but I'd prefer things such this would be optional, like Athomathon or Farportals.
Also, it never ran away of stamina, even though it kept Momentum on all this time.

He's not mixing abilities from multiple classes: The Silent Death is strictly a Shadowblade.

Hmm... You are right. But even then, high skill levels (9.6 Blinding Speed, 8.4 momentum, 6 Essense of Speed) to already op skills of op class is crazy.
Also, it indeed run out of stamina.

Is it always this strong? I don't remember having much problem with it on Brawler.

The Slime Tunnel bosses are actually single-classed randbosses: The base creature and class are constant (so the Silent Death is always an Ultimate Gwelgoroth Shadowblade, while the Neverdead is always an Archlich Necromancer), but the particular talent levels can vary. You need to have a variety of contingency plans when you go in there.