The only enemy in the fight is Bahamut, and he is pretty beefy in stats. He has no physical attack type resistances, but all elemental resistances at 40%. The fight has multiple different elevations, so you might not be able to deal consistent damage using Calculators.
Bahamut often uses Flare, but can also use Lightning, and Snowstorm. When the countdown timer reaches 0, it will use Megaflare, which deals damage in an area.
The fight will be a lot easier if you have a magic tank. Units like Rain, or Vinera (Winter) with Moonlit Fang, Fenrir works quite well here. Use taunting skills on your tank(s) and move them to the other side of your party so you don't get hit with area attacks.
Bahamut has a lot of HP and no resistance to poison. You can use skills like Bio, Bio Blade, or even Malboro esper, to poison it and deal high damage over time, then wait it out. If you want to deal more damage to it, you can get some light killer from The Perks of Having Charm VC, or some esper killer abilities from Niv Lu / Creysse.
Most units with a Spellblade job can use Bio Blade. You can even win the entire fight using just using tanky units and keep spamming poison attacks to win. Otherwise, you can use units like Rain and other units in your party be damage dealers.
Most damage dealers will probably work about the same since Bahamut has 40% resistance to all elements. Dark damage dealers will be able to deal more damage, but can get one-shotted, which can end up missing the "No KO'd Units" unless you bring a revive.