There is nothing worse in a Pokémon battle than when you encounter a Pokémon that you can't find a weakness for. Typically this happens because you don't understand the opponent's types. This happens most of the time because multiple types are put together which usually wouldn't work in most situations. In Pokémon Brilliant Diamond and Shining Pearl, one of these most common confusions comes from Whiscash, who you'll see a couple of times before battling the Bertha Elite Four. Here are Whiscash's weaknesses and best counters.
What is Whiscash weak against?
In most cases, you will see a fish Pokémon like Whiscash and know how to deal with it. If it's a Fish Pokémon, it should be weak against Grass and Electric, right? Whiscash is a special case of the Water and Earth type. This makes him completely immune to any electric move and the Fire, Rock, Poison and Steel attacks will be less effective. That said, it is doubly susceptible to Grass attacks. Hit him with essentially any grass move and you'll severely reduce his health. The best we recommend are Petal Dance, Power Whip, Wood Hammer, Seed Flare, Energy Ball, Giga Drain, and Frenzy Plant.
As stated above, Whiscash is incredibly weak against Grass-types. With that in mind, if you picked Turtwig early in the game and evolved it into Torterra, you're pretty good at it. Otherwise, great choices include Roserade, Tangrowth, Exeggutor, Leafeon, and Abomasnow. In reality though, if you take a Grass Pokémon that has been properly leveled against a Whiscash, you have the edge.