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First, I'd like to mention that My Pokemon Ranch would not let you restart your game and keep your pokemon (if you tried to do that, the pokemon you uploaded into the Ranch would be stuck there.)


My strategy for getting pokemon to level 100 is to find a place where multiple trainers can be battled back-to-back with minimal unnecessary waiting. (Hint: this describes the Elite Four!) Pokemon under the control of a trainer will give more EXP than a simple wild pokemon (it's part of the EXP formula) and you don't have to waste time recharging the VS. Seeker over and over.


Unfortunately, that already requires you to be able to beat the Elite Four, but that's not completely unrealistic. Simply using the Elite Four as a good source of experience to evolve other pokemon can easily push some of your pokemon to level 100 completely unintentionally.


But if you've got a particular pokemon in mind, sometimes you have an excellent option to level them up *really* fast. Basically, they'd have to be powerful enough to take on the Elite Four on their own... or at least a trainer or two. The secret is the Lucky Egg. It also really helps to have more than one game and to level the pokemon up in a game other than his original one. All of the EXP boosts will help a *lot*.


For everything else, I would level them up as much as I can and then use Rare Candies to finish the deal.


However, note that in Diamond / Pearl you don't have to level all your pokemon up to 100. The game has an option that can temporarily make all pokemon in a battle the same level! blaugh

It's an item your pokemon can hold. It boosts the amount of EXP they get. 3nodding

They are held by wild Chansey (although the chance is low.)