Just wanted to comment that I thought the info given in the strategy guides on this site is deceiving - makes this seem like an easy fight, I found it to be the hardest fight of either SoA or ToB...
My PC is a level 24 or 25 sorcerer, Keldorn, Amonen, Jaheria, Nahlia and Minsc, and I got my arse kicked repeatedly, although I sailed through the keep without reloading....
SOmeone posted on the strategy guides to use area damage spells and "watch the damage pile up" - well I used AD. Horrid wilting, Cloudkill, Cone of COld, everything under the sun and no damage to Demigorgon, my sorcerer ended up held or dire charmed.....
Plus if you kill of the Marileths right away he respawns an endless amount of Glazebru's....
Anyway I am posting my strategy for beating him on the Strategy Guides section, it is not easy, when I finally won I was down to myself (sorcerer), Nahlia and Keldorn with about 10 HP left, running around casting spells at the glazebru's, wasn't a pretty victory by any means..........