>>20832541>Bug and PoisonWow it's fucking nothing, and three of the four things I mentioned are neutral/immune to these (and Chesnaught is immune to Sludge Bomb.) This is why basically every good defensive pokemon (well, offensive ones too) is dual-typed: mono-types have shit for resistances, the only way you can get away with being a defensive mono-type is A). be as ridiculously fat as Chansey or Cresselia, or B). be Fairy or Water typed.
Plus, Flying is exclusively used as STAB. Meanwhile you have Water, Electric and GROUND resist, which are all very useful. It's somewhat niche as a defensive typing but it's very good for Azumarill, Keldeo, Mega Gyarados, Excadrill, Lando-T, Rotom-W, Kingdra and the rest of the rain crew, and a lot of other stuff.
