Pokémon Diamond and Pearl/Route 215

As soon as you enter, it starts to rain. This is a rainy area.

TM66:Payback can be obtained from one of the blackbelts.

Once you have the secret potion, use it one the psyducks to get passed them and go to Celestic Town to deliver the item from Cynthia.

You can also find here a Fist Plate (Hold item that boosts the power of Fighting-type moves)