ok first of all you don't need 100 heroes. I believe there is a game faq online that shows you how to beat it without becoming invincible. I've done it....I only wish that I could prove it. I don't remember the fight exactly but....every move omega does is in sequence....memorize the sequence and try to get perfect timing...i believe it took me 2000000 times to get it down. But GF is your friend here! When you are summoning them during the attack that goes well above 9999 HP make sure that you are summoning and it takes out your GF. You don't need them anyway. I beat him with Squall, Rinoa, and Irvine. Cause their limits are the ones that cause the most damage! Squall's Lionheart, Rinoas Wishing Star, and Irvines, Pulse or Ap Ammo. Rinoa is a must though because sometimes she will either cast wishing star or Invincible Moon! I know its random but in order to save your laguna card you have to take some chance. finally make sure that aura is plent and haste is cast. Thats it....good luck.....its damn near imossible but its always worth a try.....

Here was my combo...

Squall -100 (took forever at heaven and hell islands)
Rinoa - 100
Irvine - 100

All GF's equipped, and all ultimate weapons.