I use the same strategy of casting Aura on all my characters, except I let my characters stay at 1 HP when Ultimecia or Omega use Hell's Judgement or Megido Flame. With 1HP, Squall has a higher chance of doing more hits with Renzokuken, Zell has more Duel time, Rinoa has a better chance of using Wishing Star, etc. Seriously, with this strategy, I beat Omega right after he used Megido Flame and never got to use another attack. Of course, it doesn't work like this with Ultimecia. You MUST wait for her to say "And..." before you can kill her. Even if you reduce her HP to 0, she can still attack you and can't kill her until she says "And..."