Hmm...you need to learn how to properly exploit the junction system!

This is how I do it, but everyone has their own strategy n.n

Well...maybe I should simplify it...

I usually have Ultima and Full-Life on Elemental Defense. Ultima Weapon drops 100 Ultima stones and you can get Ultima from invisible draw points in The Island Closest to Heaven/Hell. They boost all of the elements, so if you equip those, Ultimecia's first form can't do much to you. It also makes unlocking the seals a breeze.

Aaaanyway, you want Cerberus, or lots and lots of triples. Cast triple first, then you want to give your party Haste, Aura, and Regen. Meltdown isn't a big deal, but it does help. Make sure you have the Revive, Recover, and Treatment abilities, Revive especially. If you haven't used a Phoenix Pinion in battle before, use it before you start so you can have the GF Phoenix. You'll want a high strength stat. I usually have Quake, Aura, or Meteor on that.

So...with Aura cast, just go insane with the limits. I strongly reccomend Zell, if you do that. Just keep using Punch Rush and Booya. You can hit three times in one second, more if you're good at it n.n
Also, if you wanna cheat a little, get a turbo controller and use Shot. BPPPPPPPPPPPPPPPPPPPPPPPPPT!

One last thing. Bahamut's card can refine into 100 Megaelixirs, which is really usefull in Ulti's final form, as she uses an attack that cuts your HP down to 1 a LOT.