This is what I would recommend:
--Make sure all of your protective spells are in place (my standards are armorplate, Enchanted Blade and Magic Screen).
--Move in close - if you stay back, the Guardian will kill you with his magic (rather like the guys in the temple in the SE Wilderness).
--I used Boiling Blood with both my priest/bishop and mage/bishop - great spell, does 125-175 damage per spell per round.
I thought about using an Elemental, but didn't because the Boiling Blood dispatched him in a few rounds.
Unfortunately, there were lots of Undead and assorted beasties behind him and I had the bad luck to get them all at once along with the Guardian. Banish takes care of the Undead handily and I always drive the beasties insane first and pick them off at my leisure.
Jem's right - if you hang back and edge your way into the cavern they won't all come at you at once, and you only have to deal with them in twos or threes.
Your mileage may vary.
[ 02-25-2002: Message edited by: Rhea ]