Instructions
Make your way to Route 212. Wander around in the grass until you encounter a Grimer. The Grimers on Route 212 will be between level 18 and level 20.
Use a Psychic-type Pokemon or Fire-type Pokemon to weaken the Grimer. When the Grimer's health bar drops to the red area, use a Pokeball to capture it.
Place the Grimer in your party and defeat other Pokemon to raise its level. Once the Grimer has reached level 38, it will evolve into Muk.