The reaper is an optional boss that appears in Mementos when you idle around for too long on one floor. You’ll know he’s coming when you hear chains rattling. The recommended level to take him on is mid 70’s.
The reaper has no weaknesses to exploit in battle, and he’ll always try to target your party’s weaknesses. If he can’t do that he’ll use concentrate and then cast a instant death blessing/curse spell. If you don’t want him to cast that you’ll have to cast Marakaran on yourself to force him to cast Megidolaon on your party instead. After that it becomes it becomes a battle of attrition as you chip away his health while healing yourself and reapplying the spell reflect with Haru as the fight goes on. If your party is well equipped you’ll eventually win. Depending on how well geared you are it can take 20-40 minutes.
If this sounds like a lot of work that’s because it is. But the reward is very much worth it. For killing the reaper, you will get 72k experience and he has a chance to drop the divine pillar, an accessory that halves your damage taken but makes it so you can’t evade attacks. This accessory can stack with the Firm Stance skill to decrease your damage taken to 25%. Also you get a trophy the first time you kill him.
There is a way to cheese the reaper, that is to fight him during flu season in November/December. During flu season, all enemies in Mementos has a chance of being affected with Despair, this makes it so that they sometimes don’t take a turn as well as dying after 3 turns has passed. You can do this repeatedly, it’s recommended that you have maxed your Star Confidant so you can escape with 100% success if the reaper isn’t effected by despair. Just go down one level from a safe room, fight the reaper and then go up a level to save and back down again.