The easiest way is to give the Pokemon an experience share and train against high-leveled Pokemon. You could also just put them in the daycare and ride around on your bike. If you have access to red, put the experience share on the Pokemon and beat some of his Pokemon or the Mt. Silver Pokemon.
route 205 for Pokemon diamond, pearl and platinum. the trainers have Pokemon from level 10 to level 17
Train weedle to level 7 then it becomes kakuna train kakuna to level 10 and now its a beedrill.
train... train...TRAIN...
Train your pokemon.
Somewhere near Mauville
First capture a Weedle or Kakuna then train either to level 10.
train, train and more training. or use the pokemon clone glitch and let your pokemon hold a rare candy for level up.
the price is a legendary Pokemon dialga at level 90 and the secret is train all of your Pokemon level 70 don't increas all of your Pokemon's level when its 70 you will never win train (ALL) of your Pokemon level (70) all you catch and received pokemons. (tips to train all of your Pokemon to level 70 battle the Pokemon league
You keep on fighting Pokemon until you level up.
At the Pokemon league.
you train them
Train them............