Here's what i did to beat the whole ultimecia thingy (except the first form).
Have Zell and Rinoa with you.
Have 100 megaelixers from bahamut's card. (it's not like you'll be able to use it)
Use cerberus (or just use triple) on your party.
Cast aura on everyone.
Have everyone's strongest weapon.
Make sure angelo knows all of his ablilities, especially wishing star and invincible moon.
Don't use angel wings!
Be sure everyone's stregnth is really high, and use physical attacks and Limits.
With zell's duel just use punch rush and booya continuously, none of the other comands (with some practice and maxed out str. he does over 100,000 with duel!!)
Between Duel, Lionheart, and Wishing Star, it was a breeze!

Other notes:
Make sure you've used a phoenix pinoin, you may need phoenix!
When fighting griever, keep everyone's HP higher than 3000, his shockwave pulsar does a lot.
Ultimecia's Hell Judgement will knock everyone's HP to 1 so have plenty of megaelixers!