if you want to level up your pokemon faster. give the pokemon you want to level up an exp share but it at the bottom of your pokemon list and battle and beat the elite4 in succession over and over until it is at the level you want it to be.
There is no faster way ok
Train your pokemon.
level 100
you train them
Train them............
level it up to level 21
any level up to 100
level it up
Yes they are weeker, but they level up a lot faster than Pokemon that you caught.
level up or action replay
level your Pokemon up to 80
You have to get Piplup for the starter Pokemon and level him up or trade for it.