Hey, here's how to beat Omega Weapon in FF8 (I hope you're not talking about Ultima Weapon, because if you can't beat that with characters at fairly high levels. . .I can only say you have a long way to go LOL

) It's been a year since I did this last and I'm working from memory here. . .so if you have problems, post and I'll look up my notes in my guidebook. ^^
Basically, you want as many Holy Wars as possible. Those are the items that cause your characters to be totally invincible and in Aura state for a few turns. If you did the card player sidequest, you will have 10 of them from using the Card Mod ability on the Gilgamesh card. (You can also win the same card again by playing Triple Triad again against the Card Knights aboard the Ragnarok on Disc Four, if you want to go to the trouble-and if you were smart enough to keep the Random rule out of there)
Unless you have used them elsewhere, you should also have 10 of these items from other sources. That leaves you with anywhere from 10 to 30 Holy War items.
Now, when you fight Omega Weapon, the team facing it *MUST* be the strongest team you have. I personally used Squall, Selphie, and Irvine, since that was my predominant team throughout the game, with the highest levels, GF compatibility, magic stores, and ultimate weapons and limit breaks. Under any circumstances DO NOT USE ZELL in this battle, because his limit break is too complicated and will blow the strategy apart.
Now for the good part. Omega Weapon cannot be fought without invincibility no matter how good your characters are. It *will* at least once use an attack that hits each character for 9999 HP. So, those Holy Wars are your ticket to winning the battle.
Have your fastest character use one of these. Hopefully, your weakest limit break character will be your fastest. Then, you have the other two use their Aura-induced limit breaks to pound Omega Weapon. Make sure you have one character using a Holy War every other round. If it wears off before that character's turn is reached, *QUICKLY* have whatever character is available use it even if it means giving up hitting Omega with Lionheart one time.
This strategy should win the battle very soon. The only way it will fail is if, somehow, you just have *very* bad luck and get hit with the 9999 HP attack before you can use the first or next Holy Wars (that happened to me the first time LOL

), or if your characters are so weak that their limit breaks don't cause enough damage to beat Omega before you run out of Holy Wars.
NOTE: Holy War may not induce Aura, I don't remember exactly. If it doesn't, you have to have one character cast Aura to everyone and the other quickly use a Holy War item after that. Sorry for any confusion-it's been at least a year and a half now that I think about it. ^^