I've split this post from Zaskull's thread because it could spoil things for other people who haven't got that far. When I was doing this battle I never had Doom cast on me so I can't really offer too much help aside from a rather futile "I guess you have to kill him faster". If I recall correctly, it never hurts in this battle to do some serious SAB work on him. It allows you to charge up the stagger much, much faster, and then once you have it maxed out you need to hit him hard with all three characters provided they're in the green. You can probably do it even if your allies (not your lead though!) are in the high yellows, too, especially if you have a phoenix down or two handy just in case. Definitely SAB him, though. You can deprotect, deshell, etc. him.

Make sure you have a MED/MED/*** handy (I used MED/MED/SYN to keep haste etc. up while I was healing) as you'll need to make sure that even if one person gets dazed at any point, you can quickly esuna them out of it.

For this battle, I used Lightning (lead), Sazh and Vanille. I flicked between...

COM/SAB/SYN - I used this at the start to buff/debuff while getting a few hits here and there in.
RAV/RAV/RAV - For pushing up the stagger (RAV/RAV/COM also works)
COM/RAV/COM - For when I was attacking all-out once I had a stagger on the enemy.
MED/MED/SYN - For when health went below half for more than one person, or alternatively when someone was dazed.
COM/MED/COM - For when only one person is low on health and the others are fine and I have a stagger.
RAV/MED/RAV - For when only one person is low on health and the others are fine and I need a stagger.

PS. 'sup G