Here's a simple strategy, but it requires powerful items and magic:
1) Summon Cerberus, giving all characters Double and Triple.
2) Cast Aura on the entire party, allowing more frequent Limit Breaks.
3) Use a Holy War item, rendering the party invulnerable.
4) Cast 'Meltdown' on Ultima Weapon.
5) Begin using physical Limit Breaks. If a Limit isn't immediately available for a character, simply press the 'change character' button (usually O) until the Limit icon appears.
6) Watch as Ultima Weapon perishes. Next stop,
Spoiler: Omega Weapon.
Good luck!
