In my opinion this is the HARDEST part of the Celestial Nexus battle.
The main boss (name not mentioned because it may provide story spoilers) doesn't melee, just casts ancient magic and Sleepga II. While that sounds kinda bad its really easy to handle as far as I, the WHM, am concerned.
Exoplates, his body armor (which renders him invincible until it is destroyed) is a little more complicated. It doesn't melee, but it does do a few aoe attacks for 100-300 damage and annoying status effects like bind. Also its main weapon skill, Phase Shift, has insane range, hits everyone for around 400 damage and stuns. VERY annoying, but if it was just that I could handle it (curaga III every round).
Orbitels. These things go all over the place, and smack anyone who gets hate. Since the tank can't hold these things (he has to keep Eald'narche off the mages, plus there are two of them). Also, though they only have about 500 HP, they respawn as they are killed. They hit me, a level 74 Taru white mage, for about 120-140. They hit a decently-armored paladin for about 30-50. Not bad, except that they are ALWAYS ON THE FRICKEN MAGES because the TANK CANNOT CONCENTRATE ON THEM. Their electrocharge weapon skill is extremely weak, hitting me for about 30 damage, and they don't summon like normal detector-type mobs...but they are still the most fricken annoying things in the game right now.
Sleeping them is the best strategy, but you're toast if Sleepga II gets interrupted by Phase Shift. Lullaby seems less effective. I don't know. This battle is HARD.