Probably the best way: Having friends with secret bases. The fights there grant you a insane amount of XP if he uses the "right" pokemon (mainly blissey is used).
If you don't have any friends which have the game, there is only the old option to grind the top 4. Don't forget that they level up to level 70 if you rechallenge them, so it is almost as good as le wow in X/Y.
Having a lucky egg makes exp farming much easier since it grants you more exp (as much as if you traded it with someone). You can get lucky eggs from wild pelippers anywhere.