This is pointless unless you are in Ultimecia's Castle because this is where Omega is. You should also free your abilities first, he is very tough & has the ability to have 1,200,000 HP approx.

First you'll need the Giglamesh Card, Card Mod Ability, & some GF's with St DEF (or even better Status DEF X4 with Elemental DEF X4) If your GF has Elemental Attack choose a new one, or use Amnesia Greens & destroy it because Omega absorbs all elements. If Level 5 death works on your character(s) then junction death magic to your Status DEF!!! (EX: Character level 10 - Will Die! Character level 23 - Won't Die!). Ultima Magic, Full-Life magic, Flare Magic, and Shell Magic could help because it makes an awesome defence by letting you absorb every element (if you have enough of them & Element Def X4!). One more thing that is important to have is AURA MAGIC!!! You can draw this from Siefer in a few battles, or draw it from draw points in the Island closest to Hell & the Island closest to Heaven. If you think your levels are too small these islands will help you level! They have the toughest enemies in the game.
Lets see... Only a few more things needed. The GF's Doomtrain, and Cerebrus. For Doomtrain go to the Sorceresses Memmorial & pick up the item there... The solomon ring. Get 6 malboro tentacles, 6 Remedy Pluses (With MED LV UP), and Six Steel Pipes. Choose the item & Doomtrain will join you. Cerebrus you have to battle though. When you infiltrate Galbadia Garden (Disc 2) He's in the Light Beam in the center of the room (a little ways in the building. Don't worry, you can't miss him!). Pick a good team. I chose Rinoa, Zell, and Squall. Set Zell & Rinoa (or whoever you choose, but make sure you have Squall!) with the GF Ability one with Doomtrain & one with Cerebrus.

Set Squall, and one other+ with the Item Command. Choose any other GF's you like... Make sure everyone has the Magic Command Also! Squall must have his strongest Gunblade, the LionHeart, also! Before we go find Omega you should use the Card Mod Ability on the Giglamesh card & any other cards that you think will help! Set those items (the ones from Giglamesh, Holy Wars, first!). Holy Wars let your party become invinsible for a short period of time. If you have the Laguna Card you can get 100 Hero's from it. Hero's give 1 character invincibility for a short period of time. Okay. We should be all set now! Bring the Fighting party to the Fountain & make sure they're ready! Diablos & his Enc None will help if you have it! Now bring the other party from the front, to up the steps the go in the right door. at the bottom of the steps in the next room there is a rope attached to a bell. There is another Green Circle right next to it. Pull the Rope & it will say "A Monster Roar can be heard in the distance." Run to the Circle & Switch Teams. Don't forget to switch junctions if you did that before! Run forward on the straight path (Not left or Right!). Omega will be there! Run into him to start battle.

Ok, quickly use the GF Doomtrain & Cerebrus. Leave Sqall standing untill Cerebrus does his thing & gives you Double & Triple. Don't boost Doomtrain cause it will heal Omega... The story here is that Doomtrain will give Omega Vit 0!!! With triple magic cast Aura on all 3 characters. Whoever has the item command use Holy Wars. Hit him with Limit Breaks untill he Dies. This works... I did it! Rinoa's Limit Breaks are Helpful... especially Wishing Star! If you use Zell use O X then up down (or down up?) Then do O X again. If you type fast & Zell is strong he will do massive damage on Omega! Squall is important because of his special limit break ending, Lion Heart, possible only with the LIONHEART GunBlade! When Aura & Holy War wears off (Aura first), just do the trick over. Please note Aura wont give a limit break every time. just keep hittin O untill it does work! If it doesent work for a long time. It probably wore off! Keep these tactics up & you'll do fine. If you win, you'll get a Three Stars (Not that Great). You will get some good AP at least. This is mostly a challenge for the bravest fighters of Final Fantasy 8! You'll also get Proof Of Omega in your Tutorial Section (Under Information)!