It fell down like a troll but does not die as it used to do, dunno if they are supposed to be killed by some certain damage type?

such regen mobs all have a weakness. that one is weak to cold iron.

Never mind, forgot they need cold iron weapons. :)

Otherwise you can kill most enemys with statdrain if you don't have the right weapon.

Ran into this beast as level 8, and holy crap that was a fight and a half! Truly did remind me of running into a certain red dragon in Baldur's Gate for the first time :)

Struggled to kill it for a little while too, and it kinda looked like the animation bugged out. Thankfully I had noticed it had a weakness to cold iron, and we had a +1 cold iron dagger in the bag, which was enough to kill it.

Poor Amiri perma died because she had Death's Door going into the fight. Glad we had a Raise Dead scroll, because I didn't want to repeat that fight as it was such a close call.

Horrible loot though :(

You dont need stat drain even.
Every character has "Coup de Grace" ability for that.
Just say "FINISH HIM!" and perform fatality :).

Interesting. So that works for enemies like this too, and presumably trolls, even if you don't have the "correct" weapon to kill them?

If the enemy does not roll the fortitude save, which would be pretty common for huge powerful creatures like trolls.

If your character has enough BAB and STR(or DEX), it isnt a problem. You can perform Coup de Grace several times until you succeed. You can also perdorm it with several party members simultaneously. After all, beaten enemy doesnt have any chance to fight back.

Coup de grace attack kill it as well as trolls.