That last fight is quite nasty, especially if you're unprepared (don't ask how I know that
.
Anyway, before opening the door, be sure everyone is at max hitpoints and mana, and that the mage types have a few mana potions in personal inventory. Put up magic screen (even if it's already up, you want it to be fresh).
On the first round, get up element shield and soul shield. If you can manage it, a haste spell, too (or next round, otherwise). Throw a silence on the gibbering head, which will force it to come up front.
Don't bother with offensive spells, everything here is highly magic resistant. Target Nebdar with missile weapons at every opportunity.
It's mainly a matter of pounding on the spooks and shooting Nebdar whenever you can. Heal All and Rest All are extremely important spells to have for this fight.
And keep an eye on your shield spells. Renew them a couple rounds before they expire (don't ask how I know that, either
. Good luck!
Nightowl2