Instructions
Travel to the flower shop above Petalburg Woods. Enter the shop.
Talk to the girl watering the berries. She will give your the "Wailmer Pail."
Go up to any Sudowoodo in your way and use the "Wailmer Pail" on it. It will attack you in a rage.
Use Water-type or Grass-type Pokemon to defeat Sudowoodo. As Sudowoodo is a Rock-type Pokemon, those two types will make quick work of it. Fighting-type Pokemon would also be effective. Once you have defeated it, you can continue.