This esper-trial-raid boss is among the easier ones when you get the hang of it’s dynamic. I’ve wasted exactly 160NRG before kicking it to the curb. Didn’t get all achievements but this very easy method will get you the win… for now, if it indeed is a bug when Megaflare doesn’t fire.
Only the very first couple of turns are important here. Buff SPR over 40% with one character and defend with the rest. This character will go down because of the Megaflare while the rest would be hanging on by some measure.
Next turn would see your party being buffed with elemental resistance via Omni Veil or some other elemental buff coverage like it. Then just attack with skills or otherwise without breaking any stat (neither full or partial should be used) Revive, heal up, and just maintain SPR up + elemental resistance and you should have it down no problems.
Shadow Bahamut has one threshold attack in the form of Maelstrom which will hit for 99%of remaining HP. Heal from this and continue the pattern to defeat the Shadow Bahamut.
See you guys on the Farplane!