lvl. 100

The fastest way is to battle trainers, because their Pokemon give you double experience points. If your Pokemon are at level one, you may have to use the exp. Share for a while before they are strong enough to fight the elite four or other trainers that give you a decent amount of experience points. You could put them in the day care (the bicycle trick when you go up the bump on a higher gear doesn't work in Pokemon Diamond or Pearl, but if you do it in the Game Boy Advanced games, you can transfer pokemon.). In the day care, 1 step gives you 1 experience point. If you run around for a few hours your pokemon's level would have increased a lot. The bad thing about this method is that you can't EV train, and the Day Care selects moves to randomly learn, you have no power over the moves your Pokemon learns until you take him back and you can't gain happiness if your Pokemon evolves on happiness. If you get them to level 100 this way and they haven't evolved, they will not evolve (unless they evolve by using a stone or by trading).If you are EV training, you can use stat enhancing items (iron, calcium)unstead of fighting wild Pokemon to increase EVs. There is a limit to how many EVs you can gain from items that is not the total limit of EVs. There is no way to level a Pokemon up to level 100 really fast unless you use an action replay or hack in another way. It will always take a very long time to get any Pokemon to level 100.

For the weaker ones (20+below) use exp. Share in places where it'll get lots of exp.(gyms, elite 4, victory road, etc.) 4 21 + use rare candy and training or you won't get the EV points.