What's the best way to gain levels with strong pokemon?

Then pick other usefull pokemon...of course they won't allow legendary Pokemon because some legendary have a very high EV's...

No legendaries don't have high EV's cyber john. Message to anyone who reads this, cyber john is an absolute moron don't ever listen to what he says. Best way to gain levels is to either find a place with a whole bunch of Pokemon who give resonably good experience and are mostly weak to the moves you have on the Pokemon you want to train, or to go to a place where really strong wild Pokemon are, I.e sky pillar or victory road and just use an Exp share until the Pokemon whom you want to train is strong enough to kill stuff by itself and thus gain experience twice as fast as it did with the exp share. To beat the brains make sure you know what they have, and have what they are weak against. Be sure to have variance in your type, 3 dragon/flying types wont always win, and will likely never win against ice beam.