I've found a nice area on Route 13. If you head for the man who runs around the trees, then keep heading south across the land bridge, there's a little patch of grass there; wild pokémon are often around level 48.
I've had loads of Pokémon like Solrock and Drifblim that keep using moves like Embargo or Swallow (without stacking anything so it does nothing) that are super weak against a level 40 Deino's Dragon Pulse.
Three battles in, and my Deino hadn't taken any damage at all, but had already gained a level. :) But don't forget to take a Lucky Egg! Those things are like EXP gold.