Ni No Kuni 2 - Tainted Monster #60 Humphrey Boss Fight. This is the strongest tainted monster in the game and unlocks the "Executioner's Ring".

There are 60 Tainted Monsters in Ni No Kuni 2 Revenant Kingdom. The last 10 only spawn after having defeated the first 50.
This boss is level 90. After having completed everything else the game has to offer you'll just end up at level 75. To grind out the remaining levels you can repeat the final story boss. Lock on to him, attack with fast strikes (square-button), skip the ending cutscene, save game, continue, repeat.

Ni No Kuni 2 Infinite XP - Endgame Leveling Exploit Method: [ Register or Signin to view external links. ] .

For this fight, put the tactic tweaker to more fire resistance and more damage against tainted monsters. Leander's Gorgon's Needle skill works best. Let it charge to 100% then release it on the boss. It has a high chance of stunning him. Use weapons that have A-rating for charge build-up (so they charge faster). Also consume a Rosehip tart (can be cooked in Evermore, makes your charge fill up faster). If you do it well you can stun-lock the boss for long durations of the fight.