Have your low level Pokemon hold a lucky egg in area 10 (make sure you have a high level Pokemon with you, then have that low Pokemon first in your party thrown out against the high leveled Pokemon you're vs'ing. Switch to your high level Pokemon and watch the magic. Make sure it's on challenge mood too.
I get over 20,000+ ex in my black 2 by having the White forest key :). I just annihilate blissey's. The most I ever got is when my lvl 1 bulbasaur defeated a level 82 Blissey with trade boost and lucky egg boost? I got 100,000 exp!
I just had my cottonee use endeavor while it had a focus sash, let it defeat my cottonee, switch to my lvl 1 focus sash Ralts, used trick room, let it faint and got my bulbasaur to defeat the Blissey. He got 46 level ups!