We all know that the Reaper is one of the hardest optional boss in the game. In order to kill it easily, wait for a day with an Influenza Warning. Note that November thirteen, fourteen and fifteen and December eight and ninen are the only dates that will have this kind of weather. When you're on one of this dates, travel to any regular floor of Mementos and wait for about five minutes. The Reaper will spawn and will come directly towards you. Make sure to ambush it so you can easily escape when the conditions aren't met. When the battle begins, be sure that the Reaper is inflicted with a Despair condition. If this does not occur escape and approach it again until its inflicted with a Despair condition. If his inflicted with this status it won't be able to attack and after three turns, it will die automatically. You will earn a large amount of EXP after the battle. You can then go to another floor and repeat this process. You will be level ninetynine in no time.