Okay then, here's my way (party included a paladin, a warrior, a barbarian, a priest, a rogue and a wizard). I walked down into the auditorium and used the toad trick (bump into a NPC as if to push them back, as soon as they begin to be pushed move backwards and they'll follow you.I backed up on to the left hand raked seating area to about right next to where one of the priests would appear. She followed me, ranting all the while, and as she finished off I threw up an all out party bless, general armouring enchantments (the lvl 1 stone one below Armoured Realm), threw up Haste spells and Lure on the Warrior and the Barbarian because they where the toughest (back then)and got ready. As soon as the two rows of priests appeared I layed in to the nearest one whilst simultaneously casting that softening spell on Elyssa. Having worked my way through one row of priests I was back by the entrance and quite a few foes down. I fled turning left (this was all back in September/December so if I get a bit vague after this please forgive me). Backing up at the corner I began an all out party healing session. I then spent a lot of time running backwards and forwards between those two side tunnels to make blatant surprise attacks in the rear of her party(healing as I went). I only had to use two or three scrolls of life (they where only about 7th or 8th lvl, might have been nearer 9th actualy) and most of my potions. I relied on getting the cultists between her and me because monsters in this game don't get hurt by each others spells much (even if we get hurt by ours
) but they do block them nicely. The priests can generally manage to cast poison bite past each other in those narrow passageways but as soon as you see her (or any of them for that matter) preparing to hurl another insect swarm duck around a corner because you'll need to heal up. Eventually you can get them down to her and maybe as many as two priests before you should even consider an attack (but if the oportunity arises don't ignore the chance to land a swift blow). In this manner you should be able to survive. It sounds like you're using a much higher level party than mine so you should be able to manage it.
------------------
Balgin, the Dwarf