9. The Grim Reaper - Persona 5
Persona 5 is such a monstrous game that it actually comes as no surprise that you and the rest of The Phantom Thieves can take on The Grim Reaper itself.
However, you might want to slow your roll a little and set that cat car to creep as the Reaper in Persona 5 is no joke. If you dawdle in Mementos for too long, you might begin to hear the rattling and scraping of chains, and if this is the case, get out of there! This audio cue signifies the Reaper is about to appear, and once it does it will stalk the player throughout the level until it catches them.
The battle itself can be brutal because the Reaper has several high level abilities, ranging across nearly all elemental types and can of course, use a skill that has a medium chance of killing the entire party INSTANTLY. However, notice I said "can" be brutal, for if you return to battle the Reaper during flu season, it'll be significantly weaker and can even succumb to Despair which kills it in three turns!
That being said, the original battle will push you to your limits.