The VS. Seeker is a key item that allows you to battle anyone in an outside area again, over and over. Using the VS. Seeker has a chance of anyone on screen wanting to battle you again, and it can be used after walking 100 steps again. Here are some places to train ingame in Diamond and Pearl with the VS. Seeker.
Ace Trainer Ariana
Ace Trainer Ariana is on Route 228 in the sandstorm area. She's to the north, right before the gate to the next area. She has an Ambipom Lv. 59, Chansey Lv. 59, and a Rapidash Lv. 59. If you battle her with the Vs. Seeker once you’ll get about 8,000 EXP. With a traded Pokémon that has a Lucky Egg attached, you'll get about 18,000 EXP. Talk about a lot of EXP!
Gentleman Jeremy and Socialite Reina
Gentleman Jeremy and Socialite Reina are at the bottom of Route 212, right before you turn to go to the Trophy Garden. They are standing straight across from each other. Jeremy has a Chatot Lv. 60 and Reina has a Roselia Lv. 60 (only if you’ve caught Heatran already). They give out little more than 1,000 EXP. unfortunately, but the catch is that they both give $12000, which is the most from any trainer in the game. Combine this with Pay Day or an Amulet Coin and watch the money come in!
Those seem to be the best trainers to battle if you need a lot of EXP. or money, which is always good.