The most important thing to remember is, that Omega has a pattern. And the Defend command completely negates the effects of any attack, Terra Break included. If you can time your use of the Defend command, you can last forever against Omega.

I'm sorry that I can't be of more help. I'm more of the type of RPGer that likes to completely overpower my opponent, and I haven't beat Omega without Holy Wars (that's not to say that I haven't tried 40+ times, before I knew about the Defend command).