after u beat the E-4 u have access to a whole new place
after u get the ppl to move the ones blocking your path to the Resort Area on your way to
the resort area there is water 

so give your pokemon expshare and surf or fish in the water(use Super Rod)and battle those
pkmn and watch your pokemon's level grow 

you can also train in the sandstorm area because grass beats rock and ground.and there is
even a pond in that area for you train your pokemon 

and if you continue even further you will run into more water where you can surf or use
your super rodand battle those pokemon until your pkmn grows stronger