There is a pretty simple way of downing him - just cast aura, drink holy waters and then use limit breaks to bash him to death. XP

Your HP must be at 9999 to survive his Megiddo Flame, which is one of his first attacks including Lvl.5 Death. Make sure your levels aren't a multiple of 5 or simply junction Death to your ST-DEF for those that require it. Then summon Doom Train or cast Meltdown on him, both methods reducing his Vit down to 0, but Omega might be inflicted with the Blind status if the former was used.

Next, you'll either want to gulp down Holy Wars or Hero Drinks to ensure immunity against all of Omega's attacks. The problem with this is, you can't cast Aura while they're invisible. Therefore, you could arrive to a situation where Omega will attack you with Terra Break or Megiddo Flame the moment the effect of invisibility wears off, forcing you to use Holy Wars/Heroes w/out Aura. Unless you're at critical status, No Aura = No Limit Breaks.

One way of ensuring that you can pull through Terra Break w/out invisibility is by using the Defend command prior to that attack. That way, Terra Break does no harm to the person that used it and you can enjoy the benefits of Limit Breaks in every turn so long as Aura has been cast, with knowledge that Terra Break always comes after Gravija.

If you're a master at executing Zell's Duel Limit Break combos, you can enter the battle with Zell and Squall at 1 HP and end the fight in a few turns.

Or you can simply call it a day, and make Selphie cast "The End."