My suggestion is Nimbasa City, in the Big Stadium and Little Stadium you can fight trainers around your level Pokemans and get some nice experience + cash from the players too.
Another place is (whether you're playing Black or White) just outside your games featured place (white forest/black city) to the north there's that path that goes from west to east. Ride on your bicycle back and forth in front of the grass on either side to make shaking patches of grass appear where happy unsuspecting Audino will pop out to fight you. Kill them to get HEAPS of exp for your favorite pokes.
WARNING: If you're big into EV training, Audino dish out 2 HP EV's per kill. I suggest fully EV training for desired outcome before slaughtering these huggable pink flop-eared dopes.