Get the lucky egg. Fight audinos. Or even better fight audinos with the lucky egg?
Or i've heard put it in the day care and then go to the battle subway. Just run around that for a long time and then it should be level 100. But who said its moves would be great....
Current Pokemon Playthrough:
Soul silver: Badges 8 currently training swinub to be a mamoswine and also be level 40 before taking on the Kimono Girls
Pokemon White: Badges 5 Currently training my whole team (have six of them) to level 40 and already 3 of them are level 40 before I beat skyla
Pokemon black 2 (but the mains are soul silver and white): 8 badges and bet the champion Currently trying to get shiny pokemon,legendary and level up my pokemons!