You need to debuff the second form if you aren't already. Don't bother buffing though, he'll just remove them. Then you pretty much just need to Stagger it and whack it as hard as you can for as long as you can. Having a dual or triple Med setup for rapid healing is recommended, as is any sort of debuff-resisting accessories you have, Poison and Daze protection in particular since those are most likely to get you killed. Merciless Judgment takes off a set amount of HP so just equipping a ton of the HP+ bangles won't help you that much, though Hope and probably Vanille will benefit from one or two if you have them on your battle team.

Keep your HP above 90% at all times so Merciless Judgment won't straight up kill you, have TP available for Renew and/or keep a healthy stock of Phoenix Downs.

~DragonHeart~