Yeah, this is a tough one. I have died often there, usually when my fighters are turned against me.
Do the usual stuff - haste, bless and shield everyone before you go up those stairs. Try to take out those magic users first, esp the priests. One technique I tried was to change my party order so that the magic users went first as they have higher mental resistances. This way you start the first round with the whole party under your control.
If all that fails, there is always the character editor! I have been known to use it to healand energise my characters before that fight.
Oh and you don't actually have to go through that room to complete the quest, you can usually kill the slith leader using spells and missiles from the entry level as he comes out when you walk past. But given that you are where you are unfortunately you have no choice about going through that room now. Sorry.