Good places to train Pokemon to lvl 100 in Diamond/Pearl?

Question

I dont want to use rare candies.

Answer 1

The only place I can think of is defeating the Elite Four over and over again.
If that doesn't give you enough because it's easy don't take your full team.
By that I mean go back to the Pokemon League put some of your Pokemon
in a PC and challenge the Elite Four with as many Pokemon as needed.

Comments

I have considered that but Ive beaten the elite4 at least five times and its such a hassle.

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Putting pokemon in the pc lowers friendship.

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Not in pokemon diamond