All you have to do, is go and find a really strong trainer ( maybe even the Elite Four ) and send out your really weak pokemon that you want to level up fast. Before the trainer attacks, switch your weak pokemon to a really tough pokemon. Then, use your tough pokemon to attack the trainer's pokemon and defeat it. When you are done with the battle, your tough pokemon will get exp. points AND your weak pokemon will get a good share of exp. points too! It will level up until it finally stops! Then, just keep repeating the process until your weak pokemon becomes a powerful pokemon.