You don't need decoy or reverse. It just makes the game longer. Just do the following:

1. Equip bubble belts to main fighters
2. Arise #1 gambit

Have everybody attack and throw in "expose" every now and then coz he gets really tough.

That's all!!!

My 3 fighters were level 99 and had the following weapons.

Vaan - Ultima blade
Basch - Ragnarok
Ashe - Save the queen

I killed Omega in 17 minutes. When I first used the reverse/decoy method it took longer.