The easiest way to beat it is by using:

-Aura
-Eden
-The End

Start the battle by casting Aura on Selphie, then have the other party member summon Eden; unless you have like 100 compatibility with it, you should be able to summon it in time.

Now you have a few minutes to get The End in Selphie's slots, and when you get it and cast it, it will take priority over Omega's attack as it is a limit break. This beats Omega in a few minutes... but may not be the way you'd want to beat it.