Items you want before the fight:
100 Pulse ammo (best gotten on disc 3 from a fixed fight in Ester City, refined from energy crystal)
atleast 10 holy wars (easily gotten from refining the gilgamesh card)

characters:
Squall, the lionheart weapon
Irvine, any weapon
Zell, any weapon

spells:
Meltdown (if doing this without max leveled characters meltdown will drop the enemies Vit stat to 0 making you deal more damage.)

just before fight:
kill each of your team mates seperately and bring them back with just a regular phenix down so they are in critical health.

Start of the fight:
Have zell start off by useing a holy war which will make your party invincible, while having irvine use his limit with the pulse ammo, and Squall use his limit in hopes that he uses the lion heart.

mid fight:
in the middle of the fight after you get omega weapons health down to about 3/4th he will start useing an attack called terra flare so make sure you use another holy war before that happens because unless someone is defending it is an insta kill.

strat:
just spam the limit's and if needed have zell reuse the holy war so you dont have to heal. you should eventually be able to kill omega weapon before the third holy war wears off.