I think a good way to beat ultramercia is to use Aura on everyone so they can use their limit breaks (especially Squall and Zell because their limit breaks can do a lot of damage) Then use a holy war or use a hero on everyone so that they are invincible. Then use your limit breaks. And then repeat until you win. If not you can use Eden which does a lot of damage if I remember right. I also think that meltdown of Doomtrain works well because it drops her vitality. You could also get everyone to have good stats so you can use them all to fight her. I'm not really sure how you can get specific people to fight her or have her get rid of certain people. I forgot because I haven't played in such a long time. I would stick with the limit breaks. They worked well for me. She really isn't that tough compared to Omega Weapon or other Final Fantasy bosses.