I'm playing in the standard difficulty, probably a bit over leveled/stated from trying to get a bunch of different Ultimates, and even I got a bit of a fright during his fight.
I imagine it can be quite the difficulty spike on Hard.
Granted, I'm not optimizing for shit, not my mons moveset, or team comp, or even the Agent skill tree (I maxed the generic tree and am now equally spreading points between personalities), but still.