This is the single toughest boss in all of Final Fantasy VIII. To start the battle, go to the room with the giant bell with your second party, ring it, and then switch to your first party, then quickly run into the chapel within 30 seconds to start the battle. It is essential to have the item Holy War or Hero. You can get 10 Holy Wars by refining them from the Gilgamesh Card. Start off by casting Aura on your characters, followed by using Holy War. You really will need this item, because if you don't, you won't survive Omega's Terra Break attack, and you'll only survive Megiddo Flame with 9999 HP. You'll also need a bit of luck as well, as the last thing you'll want is for your invincibility status to expire when he's about to use Terra Break. In terms of attacking, use Limit Breaks, and if possible, Eden and Meteor. Omega has over 1 million HP, and is Level 100, so this will be a very long battle.