So, first thing first, if you can't OTK bahalut, you can reset the fight. Since it works like the dark espers preemptive attack, instead of getting 1 turn, you'll be able to get 2 turn to kill him.

Know that this method may be considered as a cheat and could get you banned.

Non cheesing guide

Cecil Our beautiful and very mainly tank prove to be really useful here since he can use Focud to give your team the SPR you will need.

• Special mention of Luluka who can act as a second healer in place of Cecil, the boss won't do any physical attack, and you can use luluka SPR buff and her heal to help you survive through the billion AoE the boss spam at you.

An Healer Y'shtola/Refia w/ DC. It's really important you have an healer capable of surviving the megaflare and that can DC curaja. You will need it to top yourself off.

Element Resistance Marie and CoD are very useful for this trial, as their lovely guard/Omni veil are NOT green magic and Bahamut will spam the shit out of a billion element.

First few turn

First: buff SPR with your Cecil/Luluka and DEFEND with all your other unit, Bahamut can, and will kill your unit through focus easily with Megaflare, the unit whom buffed SPR will die.

Second: if you defended with everybody, you should have been able to survive, DC curaja with your healer to top everyone off, if your Cecil is dead, you can ressurect him this turn with Phoenix down (or another healer actually) or let him stay on the ground, Bahamut (from my 4 tries) won't do physical attack on the 2nd turn.

Third turn and forward: Keep buff up, the boss will start spamming element, I personally did not see a second megaflare.

At 40% he will take one turn before using Maelstrom which is a 99% current HP on your party, guarding can reduce those damage, I suppose Sakura/Y'sthola mitigation can as well reduce it.

If you don't hit him during his threshold charging turn, apparently he won't cast Maelstrom/Megaflare.

Slowly chew through his health bar, while keeping buff up and running and you should be able to survive until he falls, don't forget to kill him with an espers for the pot!

Sharing Unit

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If people want to help in the comment, go ahead!