Summoning up every monster summon you can before the battle should keep a couple of them distracted long enough so that you only have to fight two at a time. And like Hickory said, throw up every buffing spell you can, including your main character's Draw on Holy Might (if you're not playing evil).

You're not alone--I found them to be the hardest fight of the game aside from Sarevok and maybe Aec-Letec.