Instructions
Press "Start" and select "Pkmn." Move whichever Pokemon is at the lowest level to the front of the lineup. Return to the game.
Locate any trainers who you may have skipped. When you pass in front of most trainers in the game, they will initiate a battle with you, but some of them can be avoided. However, by avoiding battles, you lose out on experience that will help you in future battles.
Walk around in tall grass until you encounter a wild Pokemon and enter into a battle with it. Wild Pokemon live in tall grass and will always be around for extra battles, unlike the finite number of trainers you can fight.
Select "Pkmn" right after the battle begins. Switch to the strongest Pokemon that you have and complete the battle as you normally would. By starting the battle at the front of your lineup, your weakest Pokemon still counts as being in the battle and gains experience from it.
Place your weakest Pokemon at the front of your lineup again when the battle is finished.
Repeat Steps 3 through 5 until your Pokemon are all at high enough levels for you to continue in the game.