I didn't realize what was going on in my first attempt at the boss, but I inadvertently made the Reaper undead by using one of his Spirits (of an enemy that I didn't realize was undead). The boss constantly buffing himself was manageable until the final phase of the fight. Once he started also becoming actually invincible, and casting instant death and Quasar, things become unmanageable. Not to mention also healing himself with Crimson Moon and Devour.
Suggestion: When Reaper dies, could he lose the status changes he gets from his Spirit command? I think there's a patch like that used in BNW.
Well... just don't do it, it's far from being necessary having some guy in undead status.
Btw i think it would be unfair to remove undead status obtained by rage when he dies: the main risk of undead is dying, if you remove that risk, that might be cheap