Fight gym and computer battles. The opponents you should fight in computer battles all start with "S-" and following the hyphen will be the type your current training pokemon is. For example, if I wanted to train a lvl 5 Charmander to 100, I would face the computer opponent S-Fire. All the opposing pokemon are lvl 100 and only have the move Yawn, which in this game is basically like Splash. As your fighting pokemon's level increases, the less money/exp you get.