Final Fantasy Tactics Walkthrough

It's very amusing to go after the Ultima Demons in this battle with Orlandeau and Agrias because they'll be able to kill most of them in one hit, leaving just Ultima standing. She's actually pretty easy to defeat because she's horribly weak to Holy spells, and you should have two people with Divine Ruination, a Summoner with Lich (Which isn't Holy-typed, but works just as well), and someone who can cast Holy if you've been generally following the advice of this walkthrough so far.

Ultima will go down in a few rounds, especially since Alma will, without a doubt, cast her usual buffs on Ramza, freeing him specifically up to go toe-to-toe with the false saint.

Once defeated, Ultima swells with unholy energy, ruining her visage and becoming a skeletal demon, in a last ditch effort to both save herself and kill her obstacles.