theres an exploit i found you can do to get like 10 levels before the opelucid city
attach lucky egg to the pokemon ur training (if it as an ice or rock move, ur in better shape)
battle the pokeon breeder on the bridge (if ur around level 40-45, you get about 1k exp per pokemon)
enter the house right next to her
exit the house and battle again
repeat
since all pokebreeders rebattle you when u enter the route, you get many many exp
in about 40 minutes, i got all my nigs from level 44 to 50