Instructions
Enter into Pokemon battle. You can do this in the open world environment of the game or during a scheduled tournament against other trainers. To start a battle, walk up to a trainer and press the "A" button. Select the "Battle" option to begin.
Select a weak Pokemon from your collection to begin the fight. Make sure that the selection is the Pokemon that you want to level up quickly.
Select the "Change Pokemon" option from the left-side menu as soon as it is your turn to make a move. Change from the weak Pokemon to your strongest Pokemon. You must change the Pokemon before you make a move with the weaker one.
Defeat the trainer with the strong Pokemon. Once you have defeated the trainer, your weak Pokemon will gain the experience points that the stronger Pokemon earned. Continue using this process until you have leveled up your Pokemon.