Instructions
Play the game until you reach a score of 75,000 points or more for all the pictures you have taken. Speak to professor Oak to receive the Pester Balls.
Select the Cave level and wait until you reach the area where the Zubat grabs Pikachu.
Use the analogue stick to aim your targeting cross-hair at the Zubat and press the "B" button to start pelting it with Pester Balls. The Zubat flies in an erratic pattern, so it might take a few attempts before you hit it. After you hit the Zubat with the Pester Balls, it will drop Pikachu, who will float down holding balloons. This is your chance to take a rare photo of it.