Assuming you've maxed some roles, Omega can be triumphed over in less than four minutes. Before it unleashes Wave Cannon , you cannot Stagger it; and the only debuff it's susceptible to is Slow.
It has the annoying combo of drawing you in and then follow up with flamethrower for big damage.Once it starts charging for Wave Cannon, heal up and then switch to Tortoise to soak up the damage. Now the fun begins, as Omega overheats.
Add Deshell, Deprotect and Imperil onto Omega, while Yakshini buffs you with Faithga and Bravega. When Omega is debuffed, start piling on the damage. After about a minute or so Omega recovers from overheating, but still remains vulnerable to Deprotect, Deshell and Imperil. It now also has a weakness to physical and magical attacks, taking big damage from anything.
Take it down while it's throwing the tantrum and hopefully you get the crystal. Omega comes as a Commando.
Tortoise is a must as a defensive paradigm. The usual combination of SAB and RAV can drive up the Chain Gauge while you debuff him. Finally, Cerberus should be used when staggered for maximum damage. You will need a good SEN monster like the Silver Chocobo , a good SYN like Yakshini and a good COM like Chichu/Dragoon/Twilight Odin .
VS Lightning and Amodar
Finally Lightning has appeared as well. The fight itself is hard if you go there as soon as you can, but once you're well developed, you can dominate the battlefield. Do not try to fight the Coliseum monsters until you are significantly developed as many are very hard to take down even with fully developed characters.
During the first round, remove about 15% of Light's HP and then Amodar joins. When either Light or Amodar fall, the remaining foe fortifies themselves with several buffs, and can start doing limit breaks. I love Light's Army of One. Also, until Amodar is defeated, Light can't be provoked.
I find that a good way to deal with the threat is to debuff Amodar with Imperil and Poison, then wail on him until he's gone. Light gains Haste, Faith, Bravery, and Vigilance, as well as immunity to Dispel. If you want to remove those buffs from her, you need a monster ally with Haste Feeder, and similar like Chichu. Light also starts to use Army of One, but at regular intervals, and it doesn't seem to be terribly strong.
Stagger Light, then defeat her and hope you score her Crystal. Lightning is an Early Peaker Ravager, and can be increased by 13 levels on her Crystarium. Amodar is an Early Peaker Commando. They are very strong though so it's worth it.
She's very tough to conquer due to several factors in the battle:
Can summon up to five allies (consisting of Nightblade Puma, PSICOM Warden and Azure Behemoth)
Can recover HP
High stagger point and high chain resistance
Make some paradigms with COMs that have Ruinga. We'd say COM/COM/SYN (Yakshini) works very well.
When the battle starts, Nabaat immediately summons reinforcements. The first wave is usually 2x Nightblade Puma, 2x PSICOM Warden and Azure Behemoth (but she can also summon 2x Azure Behemoth). You need to defeat these as quickly as you can, otherwise Nabaat will absorb their essences, recovering 99999 HP for each essence absorbed.
While reinforcements are out, Nabaat buffs everybody with Protect, Shell and Bravery. Pumas can debuff you with Poison, Wardens are merely an annoyance and Behemoths can really hurt you with wide swipes. The best cure for these foot soldiers is COM's Ruinga - coupled with Faith it can deal good damage to every enemy in reach.
When Nabaat uses Sneer, this is your cue - she will soon absorb the essences of any of her remaining allies and recover some HP. If she succeeds in doing so, everyone gets hit with Daze (toss Remedy immediately), and soon Nabaat will likely KO somebody with her limit break. However, if she has nothing to absorb, she spends some of her own HP instead to launch her limit break.
Revive with Phoenix Down/Phoenix Blood, heal your wounds and get ready for the next wave of Nabaat's foot soldiers. When her HP gets lower, she gets some new tricks like draining attacks and launching attacks. Continue to press the assault on her reinforcements so she can't recover HP, and then smite her into oblivion.
When captured, she comes as a Saboteur and a very good one at that.