I always go up the entire tower with Wall Rings on everybody...that might be a tip, but you'd just have to use items to heal. Other'n that, like abrojtm said, cast Berserk on him, and he won't change his wall or cast spells, he'll just hit you. However, he WILL cast Ultima as he croaks off, which will most likely kill all of you. The way I see it, ypu have two choices...I've tried both, and both work.

1.) Rasp and Osmose all of his MP out of him. This takes quite a while, I know. A long, long time. But, then he won't be able to cast Ultima and kill you all. I've tried this only once though--because it takes so damn long--and when I got him below 80 MP, even though I'd barely touched his HP, he tried to cast Ultima, didn't have the MP, and died.

2.) If you've got Pheonix, make sure somebody knows Life 3, which you can cast on yourself before you die, and it'll resurrect you. So, you kill him, he kills you, he dies, you come back to life.

Or, I guess, you could just go do other stuff until you've got the level and the HP to withstand his Ultima.