When you face a Quagsire, you will want to be equipped with the appropriate type perk so you can take it down quickly. These Pokémon have decent attack stats, so you can put yourself at a marked disadvantage if you take out the wrong party member to oppose them. Here are Quagsire's weaknesses and best counters in Pokémon Brilliant Diamond and Brilliant Pearl.
What is Quagsire weak against?
Quagsire is a Water and Ground Pokémon, so we have two very different types working together here. The first thing to keep in mind is to avoid using Pokémon Electric against Quagsire because it is immune to damage from electricity. Instead, use Grass Pokémon to take it down. While this water Pokémon can't be touched with electricity, the grass moves are even stronger against it than your average water type. The moves we recommend are Petal Dance, Power Whip, Wood Hammer, Seed Flare, Energy Ball, Giga Drain, and Frenzy Plant, but any Grass-type moves should take out Quagsire quickly. The Fire, Rock, Poison, and Steel attacks will be less effective against it, so avoid them.
Honestly, any Grass Pokémon should get the job done against Quagsire as long as it's level and healthy enough to do so. If you picked Turtwig early in the game and evolved it into Torterra, you're ready to marvel, but other great choices include Roserade, Tangrowth, Exeggutor, Leafeon, and Abomasnow.