What is the best way to train pokemon to high levels on diamond, because it takes too long battling the elite.

i have a lv. 54 charizard that i want to get to a 100 quickly without cheating. so is there any place in diamond that i can train at. any answers would help. thanks!|||battle over WFC!.... btw i need your name and friend code if we were to battle!|||Sorry,but the only way to level up fast w/o cheating is taking on elite 4 a lot :(|||Battle Elite Four x 1000000


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Basically... :P