Bahamut
Party:
Tifa: Mythril claws, max out materia slots, MP, magic, and lowered attack spell cost.
Barret: Big Bertha, max out materia slots, HP, MP recovery, attack, HP recovery when low
Aerith: Silver Staff, max out materia slots, MP, magic, MP reduction, MP recovery
Use materia to boost everyone's MP as much as possible, also boost magic as much as possible, especially Aerith's.
ATB Assist on Tifa, Chakra on Barret and the rest of the materia can be whatever you want. I also used time + magnify on Barret.
Strategy:
Kite Bahamut between Aerith and Barret.
When the battle begins, start shooting with either Barret or Aerith. When you have Bahamut's attention, switch to the other character and start attacking to build ATB. Keep doing this until you can activate Barret's Lifesaver, and Aerith's familiar. Also cast magnify haste on everyone when you can too.
Whenever Barret has 2 bars and you don't have anything better to do, use Maximum Fury.
Once Aerith has another bar, put down Arcane Ward, then build up to max ATB again. Remember to keep switching back and forth to Barret so that one can shoot while the other has Bahamut's attention.
Once Aerith has 2 bars of ATB, stand in the arcane ward and input 2 firagas. This will make her do 4 in a row. During this time, use Tifa to go in an build ATB. The firagas will interrupt Bahamut so that you can pummel away with Tifa with impunity. This will probably do enough DPS to put Bahamut into pressure state.
If it doesn't pressure Bahamut, wait until he counts down to do that. He seems to be able to be pressured more easily during that time.
Try to synchronize Barret's ATB gain so that when you put bahamut into pressure, you can fire off a focus shot, then overcharge, then 1 bar focus shot. Do as many focus strikes as you can with Tifa.
Now just repeat that pattern using either Aerith or Tifa to fire off 4 firagas in a row within the Arcane Ward while you DPS with the others. Use Barret too if necessary to nuke.
When Bahamut is almost staggered, make sure to build up everyone's ATB to 2 bars., and make sure Tifa is at chi level 3.
Stagger Bahamut, go in with Tifa first and do this combo: True Strike > True Strike > Soul Drain > Soul Drain > Maximum Fury > Rise and Fall > Omnistrike > True Strike > True Strike (You may or may not have generated enough ATB to do those last two True Strikes). This should get the stagger damage multiplier to the high 200s.
If that didn't kill him, repeat the process again.