First, if you havent already, you should battle with trainers around the previous routes, and all of them for mostly great experience.
If that's still not enough for you, why don't you just train by battling the Pokemon in the patches of grass in the previous routes too? They're fairy high level.
(By the way, by now I'm sure you're up to Castelia City, you can battle the people in the Battle Company. They're around the Lv 20s.)