Ferrothorn is one of the best checks to this thing. Steel is immune to poison.
Sadly there aren’t a lot of special attackers that can take a hit from this guy. Starmie CAN do damage, but gets 1shot by seed bomb, and doesn’t take it down from full health. There aren’t any real checks aside from Ferrothorn (and that is only when Donphan doesn’t carry Superpower, which imo, it shouldn’t do, as you can just have something for ferrothorn in another poke) so it’s pretty difficult to stop it. It’s ability is what makes it broken. It makes it so would-be checks like Skarmory and Talonflame aren’t checks, they get destroyed completely. The ability needs to be changed or it’s gonna go straight to ubers.