when i get him under half hp, the game freezes and a popup window appears that says:
Error loading type
Metadata/Effects/Spells/maneuver_mechanics/AfterImage/DemonAfterImage.ot is non-virtual and does not have a physical file.

this also happened when i stepped on the rewind traps before the boss if i was in demon form, so my guess is that the boss has a rewind mechanic as well so the game crashes immediately when it tries to create a rewind clone while i'm in demon form. hope this gets fixed soon...