They become more powerful after getting the National Dex. The Battle Frontier Island has some powerful trainers (ones that give Exp) you can face. For Wild Pokemon, try there also, or Victory Road. There are powerful people in the restaurant in the Survival ZOne you can face, all are familiar faces. On the weekend, go to the top trainer cafe in the survival area to face your Rival. There are also the Daily Pokemon Center Battles. You can also use your Vs. Seeker to train.