As you journey to be the best that no one has ever been in Pokémon Brilliant Diamond and Brilliant Pearl, you will undoubtedly encounter some Pokémon that will give you a slight challenge. If your team isn't ready for opposition or you've never heard of it before, you'll want to have a good idea of what to do. Gastrodon is one you will meet from time to time. Here's what you should do when you meet him.
What is Gastrodon weak against?
Gastrodon is a ground and water type Pokémon. This will make it more susceptible to grass movement. Moves like Solar Beam, Leaf Storm, Petal Dance, and Power Whip will quickly reduce his health.
On the opposite side, the moves of fire, poison, rock or steel will be weak against them and the electricity will not affect them at all. Also, avoid using water moves in case he has the Storm Drain ability, which will negate the damage and instead boost the Gastrodon's special attack.
Since Gastrodon is weak against grass moves, you'll be fine if you pick Turtwig early in the game and evolve it into Torterra. Another great choice is Roserade, which can be evolved from Roselia by giving it a Shiny Stone. If you've redeemed Mew, keep in mind that he can learn any TM move, including grass types. Other favorable Pokémon include Tangrowth, Exeggutor, Venusaur, etc. In short, he uses whatever Grass Pokémon you have on Gastrodon for the best chance of winning.
If you have electric, fire, and rock type Pokémon, avoid using them against Gastrodon at all costs.