Well, I thought you would have not still have either of the trilogy, but I'll help. Well for most part, make sue both Serah and Noel are at least have most if not all their abilities, so all MED/SEN or SAB/SYN, also, have a monster or two that has the best role in COM, RAV, or SEN, you're going to need those the most, weather it's building up the chain gauge or protection from Mega Flare. I'm going all this from memory and not from my guide that's in my closet, so bear with me.
For the most part, focus on each Bahumut when you can, if you are going for the beat final boss in set time trophy, then wait until Serah and Noel are maxed out. Have a lot of Potions and your best equipment, and if you have any of the DLC monsters like Lightning, Snow, or Sazh, they'll help for a little bit, but don't rely on them for long. Your set up seems fine, so just don't go for the middle one off the bat, the left and right ones will keep causing more trouble for you then the middle, Take out the left or right ones, then switch to the middle, heal when needed or debuff/buff when needed. And keep a eye out for their Mega Flare, even one along can wreak you. Rinse and repeat and you might just win the fight.
I hope that helps, like I said, this is without the guide and I haven't played 13-2 in a while, Tales and 1.5 have kept me busy. Plus limited access to my TV, long story.