It's just all you have to survive up to the point where you can start putting your strategy into place! Begin battle by equpping as many of your three as possible with Initiative, especially one with 100% affection towards Cerberus. Make sure he goes first before the others take their turns. If you think you can hold on until Cerberus has been summoned, wait, if not, use Defend. Once Cerberus has been summoned, immidiately cast triple meltdown, aura and regen. Once Medigo Flame has been cast, spend your next turn either defending or healing, make sure to have at least two people defending in time for Terra Break.
Sometimes, you get an extra turn in which to do this if Omega casts lv5 Death (100xDeath to Status J should guard you entirely), which will make the match easier. Once you get into a system, have one person constantly defending or re casting magic and then if needed healing. Eventually if you punish him with Duel (booya punchrush or punchrush booya constantly) and a few Lionhearts...he should crack, eventually