I Was Just About To Fight The Elite 4 In Platinum with My New Darkrai(From my friend:), but as i started to fight, i found the Elite 4 Were almost 20 lvls higher! When Did This Happen, and Where Can I Train My Team To Beat Them?
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.
If you want to trai ur PKMN I suggest Iron Island victory road and on the way to victory road from sunnyshore city
Stark Mountain has high-level Pokemon-and strong trainers. Try catching Pokemon there too, like MAGCARGO, WEEZING, and RHYDON.