Try Nimbasa's Sports Stadiums. The trainers there are at a decent level depending on where you are in the game. However, you can only challenge the trainers within them once a day, and there are times when you can't get in the stadiums at all because of a game going on.
There are also Pokémon breeders dotted around Unova, who will always battle you once when you enter their locations; they're in Lostlorn Forest and Routes 4, 6, 7, 11 and 22.
Or you can try grinding with Audino; just run/bike around a field of tall grass until a patch shakes; when you enter it, there's a good chance you'll run into a wild Audino.
You can also battle in dark grass, since the Pokémon there will have a higher level compared to ones found in regular grass, plus there's a chance you'll be put in a double battle.
Each of these tips are better when your Pokémon is holding a Lucky Egg.