Instructions
Travel to the National Park in Johto. Find a break in the fence in the northeast side of the park, and pass through into the trees.
Follow the tree path until you come to a rock wall. Use a Pokemon that knows RockClimb to scale the wall. A small clearing is at the top.
Use a Pokemon to headbutt the tree at the top of the clearing to knock out a Cherubi. Battle the Cherubi to lower its health points. When its health bar is blinking red, throw a PokeBall to capture the Pokemon, and add it to your collection.