Congrats
About Omega..
The idea here is to beat Omega before he unleashes Terra Break (Terra Break should be his 5th or 6th turn) and without using Heroes/Holy Wars.
FYI, Terra Break is a really cheap move that sends some 10-20 star-like objects falling down on your party, each hitting one character for 4000 damage (regardless of defense) or 2000 if you have Protect on. The Defend command cuts physical damage to 0, and it does work on Terra Break. Without using Defend, you will probably only have 1 character alive after Terra Break when using Protect.. if you don't use Protect.. well.. you're dead. It's also possible to protect against it by summoning high HP GFs (the GFs will take the damage instead, and probably die too).
Preparation:
* 9999 HP on everyone
* 100 Triples junctioned to Spd (Speed) on everyone
* Spd+40% on everyone (you really need as much speed as possible)
* 255 Str on Squall and Zell (Zell most importantly)
* Battle speed set to the
slowest
* 100 Death junctioned to status defense if level is divisble by 5 (like 100)
* Get as high Vit and Spr as possible with whatever magic you have left to junction. It's not completely useless, it will help against Meteor and his physical attack (if he does one at all).
* Put Cerberus on the character with the best compatibility with him (if needed you can boost the combatibility by using Dragon Fins on the character, one Dragon Fin will raise the character's compatiblity with Cerberus by 3)
* Put Initiative ability on the character with Cerberus (would probably have to be the 3rd party member besides Squall and Zell then, because that party member doesn't need 255 Str and therefore has more room for customization).
Strategy:
Use only limit breaks with Squall and Zell once you are set up with Haste and Aura. With Zell's limit, only use the 2 simplest moves, Booya and Punch Rush I think (they will repeat indefinitely). If you have 255 Str, each hit will do ~5000 damage to Omega, 9999 if critical, and since those 2 moves are so easy you can get in over 40 of them in one single Duel, hitting up towards ~250k per Duel in total. That's why Zell needs 255 Str more than Squall does. The 3rd party member should mainly be prepared to heal (only heal when explicity stated so) and refresh Aura when needed, but can attack in some way too if you like.
The battle:
1) Omega will start the battle with Lv5 Death, but it should miss everyone. Whoever has Cerberus, summon it ASAP! Have the next character with a turn cast Meltdown on Omega to cut his Vit to 0. The last characters should wait for Cerberus to finish, so they get Triple:d.
2) Once Cerberus has been summoned, cast Aura and Haste on everyone using Triple ASAP. From then on, Zell and Squall should only do limit breaks,
especially Zell.
3) Omega's next turn should be Meteor (it's possible he throws in a physical hit in between Lv5 Death and Meteor though, happened when I fought him actually). Heal immediately after Meteor, preferably with the 3rd party member (that is, not Squall/Zell).
4) After Meteor, Omega's next turn will be Megiddo Flame. It will deal 9998 damage to everyone regardless of their current/max HP or defenses, so everyone should be left at 1 HP (if anyone died, you failed), so it's crucial that you heal after Meteor. Do NOT heal after Megiddo Flame. I repeat, do NOT heal after Megiddo Flame. Omega's next attack will be Gravija, a gravity attack, and it will deal 0 damage to everyone when you have only 1 HP. Plus, having 1 HP helps your limit breaks. Squall will be more likely to get more hits with Renzokuken, and Zell will get more time for his Duel. Heck, you don't even need Aura at 1 HP
5) From the moment Omega has used Megiddo Flame, just hit away with all you got. After Gravija, his next attack will be Terra Break, but if you played your cards right he will be dead before he uses it.
To sum things up...
* 9999 HP, 255 Str (on Squall/Zell) and as high Spd as possible.
* Summon Cerberus for Triple on everyone.
* Meltdown Omega and Aura/Haste everyone.
* Pound away with Renzokuken and Duel, heal ONLY after Meteor.
If you chicken out and get scared after Gravija, you could heal up and pray that you survive Terra Break (taking appropriate measures, like casting Protect, summoning GFs and/or using the Defend command), and try to finish him afterwards (provided you survive, you will likely have only 1 character at most alive after Terra Break unless you use Defend or summong high HP GFs in time). It certainly is possible. With high enough Spd, 255 Str and proper use of Duel, you will be able to kill him before Terra Break though.
Also, it's possible casting Triple manually rather than summoning Cerberus would be better, so do as you want there, but I think I rather summon Cerberus (with high compatibility of course).
EDIT: One final note.. you may want to practice Duel before fighting Omega, unless you've chained Booya and Punch Rush before that way (which is commonly called Armageddon Fist for some reason). Duel will be your main damage dealer, so you don't want to fail that part.
any advice is much appreciated! 
