Hydreigon is a fantastic check to M-Mawile if given a free switch (and provided it isn't above +2).
It either forces it out without taking damage or if M-Mawile decides to Sucker Punch, it dies.
+2 252+ Atk Huge Power Mega Mawile Sucker Punch vs. 0 HP / 0 Def Hydreigon: 179-211 (55 - 64.9%) -- guaranteed 2HKO
252 SpA Life Orb Hydreigon Fire Blast vs. 252 HP / 4 SpD Mega Mawile: 317-374 (104.2 - 123%) -- guaranteed OHKO
In fact Hydra is a pretty good poke in the meta right now, mostly due to its typing and synergy with such a large portion of the metagame. I've been wondering when its usage will increase, since it can check so many pokemon that are metagame-relevant (most notably Rotom-W, Talonflame and Aegislash).
Quoted for supreme truth. I think the most common set is the Weakness policy one though. What's the sub toxic set?
While it's certainly works as a check to megamaw (unless you like scarfed timid hydrei, which is screwed), it does'nt really check aeigis all that well tbh...
252+ SpA Life Orb Hydreigon Dark Pulse vs. 252 HP / 4 SpD Aegislash-Shield: 257-304 (79.3 - 93.8%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Aegislash-Blade Sacred Sword vs. 0 HP / 0 Def Hydreigon: 262-310 (80.6 - 95.3%) -- guaranteed 2HKO
252+ Atk Aegislash-Blade Shadow Sneak vs. 0 HP / 0 Def Hydreigon: 44-52 (13.5 - 16%) -- possible 7HKO
When you factor in LO recoil, this means that aeigi can actually take the hit and KO back if it's fully invested in attack. For more special oriented sets...
0 Atk Aegislash-Blade Sacred Sword vs. 0 HP / 0 Def Hydreigon: 202-238 (62.1 - 73.2%) -- guaranteed 2HKO
0 Atk Aegislash-Blade Shadow Sneak vs. 0 HP / 0 Def Hydreigon: 33-40 (10.1 - 12.3%) -- possible 9HKO
Things are looking much better for hydrei. Hydrei is taking somewhere between 80-95% (remember, LO recoil), but it wins, if only barely.
specsdreigon has a decent chance to ohko, if you really want an aeigi check
252+ SpA Choice Specs Hydreigon Fire Blast vs. 252 HP / 4 SpD Aegislash-Shield: 270-318 (83.3 - 98.1%) -- guaranteed 2HKO after Leftovers recovery
As for checking TFlame, he can only really swap into a tflame locked into Flare Blitz, he cannot take priority brave birds well at all. He can KO though if given the chance, relevant calcs.
252+ SpA Life Orb Hydreigon Draco Meteor vs. 4 HP / 0 SpD Talonflame: 399-472 (133.8 - 158.3%) -- guaranteed OHKO
252 Atk Choice Band Talonflame Brave Bird vs. 0 HP / 0 Def Hydreigon: 234-276 (72 - 84.9%) -- guaranteed 2HKO
252 Atk Choice Band Talonflame U-turn vs. 0 HP / 0 Def Hydreigon: 182-216 (56 - 66.4%) -- guaranteed 2HKO
He is by no means a counter, but he can revenge kill non-SD variants fine.
He checks rotom-w fine, and rotom-w can't really do anything back. Even trickscarf isn't that crippling since hydrei's most popular set is scarf, so even if you had something else, being scarfed isn't the worst thing ever. I'd actually call him a counter, as he really doesn't fear anything rotom-w can throw at him and has a chance to ohko physically defensive variants.
4 SpA Rotom-W Hydro Pump vs. 0 HP / 4 SpD Hydreigon: 67-80 (20.6 - 24.6%) -- guaranteed 5HKO
4 SpA Rotom-W Volt Switch vs. 0 HP / 4 SpD Hydreigon: 42-51 (12.9 - 15.6%) -- possible 7HKO
252+ SpA Life Orb Hydreigon Draco Meteor vs. 252 HP / 0 SpD Rotom-W: 278-329 (91.4 - 108.2%) -- 50% chance to OHKO
sp defensive variants fare a fair bit better obv, but they still take a beating.
TL;DR I agree with all but aeigis, most hydreigons have a rough time with smogonshield.