What is theQuickest Way to level up Pokemon in Diamond and Pearl?

Answer 1

You could either find a daycare for Pokemon or look around for rare candies (which may be hard to find). The only other way is to battle a lot of Pokemon, either wild Pokemon or other trainers. You can put your weakest Pokemon above all your other Pokemon and challenge a strong Pokemon with lots of exp. points. Your weakest Pokemon will come out first but then you can switch it for a stronger Pokemon and win the battle. This way your Pokemon gets points without doing any battling or getting hurt.