Do I get to say I told you so?
Anyways, Donphan has a lot of things that make him inferior to Groudon and Terrakion. Whenever I face a random Donphan, I always go aggro Zekrom EX and then aggro Mewtwo. Donphan cannot OHKO Zekrom EX like Terrakion can, and Zekrom EX only needs to use 1-2 Pluspowers to OHKO until Donphan's bench damage have made them unnecessary. Because they are Stage 1s, a prize trade will always be in my favor. Once Zekrom EX is knocked out, Mewtwo EX just cleans up, only being 3HKOed by Donphan but harder to set up. If the Donphan player has one of his own, then I'll KO that one with another Mewtwo. He should struggle to get the 2nd if he even plays it, especially if you N him to 2.
Donphan has a pretty good CMT matchup though, and I think it's Darkrai matchup would be ok.