Instructions
Enter a Pokemon Center in any town, if you don't yet have Metagross in your active party. Access the PC in the back of the room. Open the Pokemon storage box, and put Metagross in your team.
Pause the game and move Metagross to the first position on the team. This ensures Metagross comes out first at the start of a battle.
Start a Pokemon battle by walking through tall grass. It's also possible to battle with trainers by talking to them, if you haven't already defeated them. When you defeat a Pokemon, Metagross earns experience points.
Keep battling Pokemon with Metagross until it reaches level 53. You receive a message that Metagross wants to learn Meteor Mash. If Metagross already knows four moves, choose a move for it to forget. Meteor Mash replaces that move.