Here are some ways:
1.) Talk to and battle trainers in Big Stadium and Small Court.
2.) Find Pokemon in the dark grass around Southeastern Unova(areas around Nacrene City, Striaton City, etc.). The Pokemon there are pretty high-leveled.
3.) Some shops in Join Avenue have services that can level up your Pokemon.
4.) Check Hidden Grottoes every now and again. You might find Rare Candies!