Well,there's always AUdino, but they're pretty hard to find...
Hold Lucky Egg; it will give you more exp.
Route 10 and Victory Road will help as well. Also do EVs if you don't.
( [pokemondb.net/ev] )
If you haven't, beat Tornadus/Thundurus and Cobalion, Virzion, and Terrakion. (Cobalion must come first)
If you beat the E4, they will come back! :)
Just my two cents. Hope I helped!