Omega is immune to most status ailments, except for slow and berserk, but it has no resistance to all attack types and magic. A -20% weakness to Light since it's Dark element.
Omega will start the fight with a DEF increasing skill that lasts for 3 turns. This can make it difficult if you are fighting against it using phys attacks. Recommended to bring a shield breaker units.
Omega can inflict Paralyze from one of its attacks, and its counter move, "mustard bomb", can inflict sleep.
If you use a physical attack type team, it's recommended to bring units like these to disable the barrier. This can really be the important piece that's needed to win the fight, otherwise you'll be dealing very little damage every turn.
Yuna and Sakura are strong Light mages that can keep dealing damage without having to worry about its physical barrier.
Any unit with Steal Time ability from thief main job or sub job works here as well. It doesn't have to be a UR unit, so there are plenty of good options that you can go with.
Since there's only 1 enemy, units with skills that can reduce an enemy's ATK and/or MAG can really help with survivability.