First, enter a 1 player mode(use Training Mode) or VS mode game, choose the characters for the game, and pick Saffron City to be your stage at the "Stage Select". In Saffron City, there is a box-like object that will have Pok'emon attacking from inside it. Pok'emon like Polygon and Venusaur will be seen. They can give you some damage, but the Chansey actually gives you life- 5 percent off. To get the life, run up to the Chansey when it pokes its body out of the box.

Note: This does not work on Chanseys who come out of Pok'eballs.