heres a way to beat him before he uses Terra Break:

1) Have Rinoa, Squall and Zell in your party
2) Make sure they all have full strength (especially Zell and Squall), full HP's, really fast speed (try to get +20 and +40% to all), and you might want a good luck on Squall so he can get more opportunities to get the Lion Heart finihser in.
3) Once battle begins, have Rinoa cast Meltdown on Omega
4) Have Zell cast aura on Squall, and then Rinoa cast aura on Zell
5) Just keep using Squall and Zell to pound Omega with their limit breaks.
6) Just have Rinoa there for healing and magic stuff

Keys to the limit breaks:

1) Only use Zell's easiest combos (up + down directional pad and circle + cross)
this way his limit break will take out around 150 to 200 thousand HP's of damage on Omega if you do it right.

Hope that helps. Just be sure to keep healing up. Do it right before he hits Medigo Flame, which takes 9998 HP's out of everyone.

Also, make sure you have Bahamuts card when you go into battle, so you can use his Megalixirs