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 Black Belts.

Keep following the Galactic Grunt from Pastoria City. He will lead you to Cynthia.

Once you have the SecretPotion, use it on the on Route 210 to get past them and go to Celestic Town to deliver the item from Cynthia.

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