It will take you around 80 hours to get to the level 60 range in Persona 5, but you can power level through the last 40 levels or so in under 2 hours.
<I made a video demonstrating the entire process but Atlus sent me a copyright strike and took the video down so shout outs to Atlus woohoo>
1) Visit Mementos during the Influenza outbreak period (13-15 Nov, 8-9 Dec)
2) Wait around a Mementos floor for about 5 minutes. The Reaper will appear
3) Engage him in battle. If he does not have the Despair status effect, run from the battle
4) Repeat step 3 until he has the Despair status. Wait 3 turns for him to die on his own, yielding over 70k exp
*Only works on each floor once. Go to another floor to spawn another Reaper.
*The Star’s Rank 9 Confidante ability allows you to instantly escape from battle. This ability is very helpful in this endeavor, and is highly recommended.
** Maxing out The Moon’s Confidante ability will allow you to gain more EXP, even faster. Make sure you max him out.
***Some users have reported much lower EXP gains. I’m not sure why, and the Japanese wikis don’t say why either.