I think the OP is talking about Dark Bahamut/Wyvern (the red Bahamut) in the last place, Gaime.
If you are, don't bother casting Reflect on your party. Dark Bahamut always starts out using Mega Flare, so heal/revive your party ASAP (hopefully Rydia is alive and can cast Asura and you get the effect you really need). Just attack it or throw stuff at the guy. Don't bother playing magic warfare and don't use summons back at it (it counters with Mega Flare, at least if you use Bahamut against it).
At level 53 he shouldn't be too tough. Do you have any Protect Rings? If you have 2, try having one on Rosa and another on Edge (if you are going to throw stuff) or Cecil (if you aren't going to throw stuff). Sometimes you can get lucky if you have Cain use Jump and Dark Bahamut targets him in the air with his Flare spell and no one gets hit. Eventually Dark Bahamut's Reflect will wear off and he may hit himself with Flare before casting Reflect again.
If you get a rhythm going, Rosa should be able to keep up with the castings of Flare with a Cure3 or Cure4 spell on a single party member, or Cure4 on everyone to keep your HP up.
Ribbons can help too, but it is a bit easier to get those after you beat this guy.