Whats up Criss to the Cross
here's you're answer.
after you beat the E4 you can try training your pkmn in Nambasa City.
after you beat the E-4 all the pkmn level are at about 50 & up.. any way its more a of a challenge after the E-4 and you have plenty of trainers on a daily basis .
There is a small stadium and a large stadium hosting different sporting events.
Also in Opelucid there are some worthy trainers you can battle daily in White they are triple battling you..
In Black im as yet uncertain but i read its a rotation battle..
hope this helps..