Rare Grooming levels up a Pokémon by 1 level.
Rare Grooming can be received from the PokéGroomer, who is located in the Beachside Resort and Casino restaurant area. Rare Grooming is at the charge of 15000. Due to his "busy schedule", he can only groom a single Pokemon once per day, so make it count.
Leveling a level 100 pokemon will crash the game.