Yes, it's possible. Make sure you have 3 characters with 9999 HP. Make sure that they have good Junctions that raise Speed and maybe even Auto-Haste. When Omega uses Megido Flame, make sure your characters' HP is full, it does 9998 damage and their HP will drop to 1. Now, just cast Aura on all three of your characters. Use their Limits like crazy until Omega dies. I was able to beat Omega using this strategy and he never got a chance to execute another attack after Megido Flame. That's right, no Terra Break!

Or you could just Selphie's The End.