how do you kill torgal and his two buddies in the keep?

[Deadalready]

I've found that when cloud kill is used it often instant kills Torgal, not sure if that's happened for anyone else.

Torgal can be pretty dangerous though, he has slow and ability drain as innate attack abilities. If you are swamped attack and kill Torgal first and avoid most magic spells (save magic missile) because he has magic resistance. Using summons to distract the other trolls will help or having someone a specialised take take Torgal's brunt.