Instructions
Make your way to Mr. Backlot's mansion on Route 212.
Enter the mansion and make your way to the Trophy Garden at the rear of the mansion.
Walk around the mansion until you encounter a Pikachu. This can take a few battles, so be patient.
Use Ground-type moves to weaken Pikachu. Do not use Flying-type or Water-type Pokemon; Pikachu is an Electric-type Pokemon, and his attacks will be extremely effective against them. Once Pikachu's health is in the red, use a Pokeball to capture him.