This is not as hard as some people might tell you, you just need to learn Omega's attack pattern and know what to do to deal with it. I'll explain the attack pattern below. Keep in mind that Omega can also use a regular physical attack between these attacks at any point.
1) Omega always starts the battle with Level 5 death. Make sure you have 100 Death junctioned to ST-Def-J to every character to avoid instant failure.
2) Next is meteor, use a megalixir to get back to 9999HP as soon as meteor is used (You can get 100 megalixirs by card modding Bahamut)
3) Megiddo Flame, the reason you need to heal back to 9999, this attack will hit everyone in your party for 9998 damage, leaving you with 1 hp. No need to panic though. For now keep hitting him with all you've got
4) Gravija - if you'd prefer you can heal after megiddo flame to stop the chance of him killing any of your party with a physical attack between that and gravija. Even if you're on 1 hp though you can't be killed by Gravija. Keep attacking him until he actually uses Gravija, then you want to use a megalixir and prepare for the next attack
5) Terra Break - This is one to watch out for. You need to have the defend ability, (Learned from Brothers & Cactuar) any character using defend will take 0 damage from the hits. It's a good idea to have revive on one of the characters with defend too, just in case you get the timing wrong and one of your characters stops defending before he uses the move. The third character without defend can summon a gf to take the hit for them although if unlucky they might still die.
6) Ultima - Not really a problem, just make sure your HP is high and keep pummeling him with renzokuken/ammo
7) Light Pillar - This will kill one of your characters, revive them asap and keep hitting omega.
After Light Pillar he rotates back to meteor and repeats the same pattern. Just make sure you get Vit-0 on him with meltdown and keep hitting him with Squall and Irvine's limit breaks. Auto Haste can be a big help in getting more attacks in between omega's attacks. Your third character ( I used Quistis for mighty guard although it wasn't really necesarry) should be supporting with Aura and phoenix downs when needed.