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You can go to seven island and go all the way south. There are two pokemon rangers, and the female ranger has chansey. Every time you beat Chansey, she gives you a little more than 3000 exp points. You can use VS Seeker to battle her over and over again.

I find that using the VS Seeker in the Sevii Islands gets battles where the pokemon are levels 45+ at least. I find One Island is the best, there's a group of trainers by the hot spring that is really good for that (I levelled up a high level Donphan really quickly there and its where I trained my Dewgong, Dragonite, Venusaur, etc. and it went pretty fast) because there's about three trainers and a double battle you can easily get in range. Since the hot spring heals you whenever you go in it, it's even faster. Although those trainers all have fighting types, just so you know.

yeah use a VS seeker in island 1 its good for training and i like the hotspring next to the trainers.

Elite 4 and victory road is the best but on islands is icefall cave and silly trainies.

the poke league is the only best place.. there are lots of tough trainer and stronger pokemon.. so it faster up quicker