XY Question: Fast leveling

[Chawizawd]

Hey guys, I'm rather new here so please forgive me if this question is answered somewhere on this site already. I'm playing Y and getting some good progress in developing some pokemon. I can breed for IVs, hidden abilities, and egg moves. I can even use power items and Pokerus during horde battles to speed up EV training. My problem is that once a Pokemon reaches around lvl 30, I hit a wall. How do you lvl your pokemon up to 50, or even up to 100? Back in R/B/Y you could just smash the Elite 4 a bunch, but in XY I think it's too slow. Am I missing something? Any help with levling would be appreciated!

Thanks in advance!