Well...Scarf Garchomp and SDef Celebi are checks to Hydreigon, but both have their problems with him (Garchomp can't switch into Draco Meteor, and Celebi will lose if it has Dark Pulse, though few do). Unfortunately, Hydreigon just doesn't have any real "counters," more just offensive checks. You have two though, so you should be fine. But Hyreigon is one of those Pokémon you sorta just have to play around.