Instructions
Stock up on ultra balls if you have less than 20 in your bag. Purchase one quick ball from the Poké Mart in Pastoria City. If you use the quick ball on the first turn of a battle, the catch rate of the quick ball is double the catch rate of an ultra ball.
Go to the Pokemon Mansion, which is located in the northern part of route 212. Enter the mansion through the front door, and walk through the entrance that is directly north of your character. You will find yourself in the Trophy Garden.
Look for a male Pikachu in the long grass of the trophy garden. You have a 5 percent chance of finding a male Pikachu. Throw a quick ball at the Pikachu. If you don't catch it, go on to Step 4.
Attack the male Pikachu with magnitude, mud shot and other ground-type moves. Poison, paralyze or freeze the Pikachu to make it even easier to catch. Go to your bag and fling ultra balls at the male Pikachu. Once an ultra ball stops wobbling, you have caught the Pikachu.