^ Bahamut > All bosses. Though then again, I raised my character's stupidly well, and Bahamut was doing like 45'000 1 Mega Flare. And then I went and got Blitz Ace before Flux. I believe that Overkilling bosses is always fun.
My stratergy is similar, but has two variations. The first is for if you got Yojimbo before Mt. Gagazet, the other if not.
Plan 1:
Set up: Tidus, Auron, Yuna. Yuna needs Holy, and you need all your Aeons to have full Overdrive bars. If Yuna has one as well, all the better. Also, a large store of money will help.
Form 1: Physical attacks from Tidus and Auron. Holy from Yuna. Keep MP and HP up. That's all for now.
Form 2: Keep up the physical attacks and Holy. Keep HP up, but DON'T HEAL ZOMBIFIED CHARaCTERS. Unless they are REALLY low health, then un-zombie them, restore their HP. Just make sure 1 character, preferably Yuna, is in Zombie.
Form 3: Have whoever is alive after Megadeath either revive Yuna, or, if it is Yuna, have her either Grand or normal Summon Yojimbo, and give him all your cash, then cross your fingers. If you're lucky, Zanmato will end it before things get ugly. Otherwise, nuke her with all your remaining Aeons. Bahamut should always be the first one after Yojimbo. Why? Mega Flare breaks the Damage Limit from the get go. I used in in Plan 2, 45'000 damage.
Plan 2:
Set up: YOU NEED YUNA WITH GRAND SUMMON. And all your Aeons with Full Overdrive Bars. Also, you need Yuna to have Holy. Finally, Tidus and Auron back up. Yojimbo though is not required.
Forms 1 + 2: Same as above, make sure to end Form 2 with at least 1 person in Zombie.
Form 3: Live through Megadeath, then either revive Yuna or have her Grand Summon Bahamut. It helps if Bahamut's really powerful. Mega Flare, then, if you can, Mega Flare again. She'll either be dead or near dead. Then, once Bahamut falls, Shiva, Ixion, Ifrit then Valefor in that order of Overdrives. If Yunalesca ain't dead after that you've neglected your Aeons.
The simple man's stratergies, aka Zanmato Quid Quo Pro and Aeon Overdrive Nuke. Not the most impressive wins, but the Overdrives look cool and it get's the job done in record time.