[DragonBice]

It's amazing that I begin with telling everyone what I have tried already and getting almost only suggestions on using spells that I know don't work.

Well, thanks for trying to help anyway, I have killed him now though. I sent Korgan and Sarevok in with their hardiness, berserking and whirlwinds and finally it actually worked. Guess I needed some luck with the attack rolls.

By the way, people are talking about Mariliths. Are you playing on a different difficulty setting? I play on the normal one and Demogorgon spawns 4 or 5 Glabrezus every 5 seconds, but no other types of monsters. Some of the Glabrezus cast spells as well.