Hopefully it gets some traction here because whoever moderates the Bug forum isn't getting the picture.
Apparently since 5.1 (the point they "nerfed" the encounter) when King Mog transitions and arrives he's doing massive damage via Memento Moogle that a normal lvl 50 at the proper item level simply can't survive.
Multiple people have confirmed it.
Pic of the damage
Video of it happening
Video from 2014 showing the same attack in the same situation only doing 131 damage (Protect and stoneskin still existed at the time and should have roughly accounted for 20% damage reduction)