You train Pokemon faster by having them hold the Lucky Egg. Also, instead of using Rare Candies right when you get them, save them up for the later levels when more experience is required. Wild Pokemon that you find in the new areas unlocked by beating the game have wild Pokemon with levels ranging from the high 40s to low 60s. The Nimbassa City Sports Stadiums provide a reliable experience gain each day. About 7 or so trainers with Pokemon in the mid 60s provides good experience. Morimoto and Cynthia can be battled, but they are powerful trainers. Cynthia is the most difficult trainer in the game to beat. Cheren can be battled everyday at the end of Victory Road once you've met him at the street performance area on Route 5 where you first met Alder. Bianca can be battled every Saturday. Both Cheren and Bianca use level 60s. The Riches can also be battled. In the evenings, the Royal Unova sets sail, and you can battle powerful trainers there. But before doing these things, just travel around the new areas of Unova, battling all the trainers you meet along the way. They use Pokemon in the mid 60s. Just by going through the game, I got my Emboar to level 100 and my Pokemon to the high 60s by the time I did my E4 rematch.