I haven't beaten Black 2 or White 2 yet, but the place I got to train in White are the Big Stadium and Small Court in Nimbasa City. The Royal Unova or the big ship in Castelia City is a another good place, you can only ride on it during evenings right at sunset though. http://bulbapedia.bulbagarden.net/wiki/Royal_Unova#Embarking_times The times it sets off. I know that in BW2, there's some level 50 + wild Pokemon in Clay Tunnel and the Underground Ruins and maybe you could train against those too. So this helps.