you can take the Reaper down really easily if you know the trick. i wasn’t planning on using it, but it all fell into place. i’ll try and beat him properly on NG+ or later as he respawns.
spoilers in case you’d rather not know and do it properly:
if you go to Mementos during flu season, there’s a chance that the enemies, including the Reaper, will start the battle with Despair and automatically die after three moves, so go in, find the Reaper, if he doesn’t have Despair run away,
repeat until he does and kills himself for you. i went in to do the game cheater request coincidentally during flu season and thought i’d do it while i was there, think everyone went up like 7 levels just from that fight and all i did was guard for 3 turns