Mastered Dark Bahumut with the following team and items using the reflect dispel strategy;
Garnet:Storm Staff, Sages Surplus, Angel Ring
Reflect, Curaga, Sky Pirates Pride
Quistis:Blitz Whip, Black Robe,Rune Armlet
Magic Breakdown, Biora R4, Impetuous Youth
Golbez:Rune Axe,Genji Shield,Angel Wings
Bioga R1 ,Biora R4, Devotion
Vivi:Lilith Rod,Gold Hairpin,Magic Armlet
Comet R5, Biora R4, Rod Master
Sazh:Keepsake Knife,Black Cowl, Gaunlets
Dispel, Mental Breakdown, Zeal
I had friends SB as Sazh with Boon.
Overall I've been lucky enough to have great gear for this battle, but my main suggestion to anybody having trouble is to cast Reflect and Dispel with two separate characters so there's no chance of messing up the timing. Haste is really important too in order to get your healer to heal each member before the next Mega Flare hits. I managed to just take two hits from Mega Flare with the count down at 2 before a third would've happened.