BTW, Hydreigon's Fire Blast is a OHKO against ferrothorn in rain after stealth rock damage. Even without it, Ferrothorn will be left with pretty much no health.
Here is the damage to 252 Hp 252 spdef Ferrothorn in rain from Timid Life Orb Hydreigon after Stealth Rocks, which is 330 hp. 66.7% - 78.8%
Without taking damage from Rocks 62.5% - 73.9% I used Forretress with +2 Sp Def and enough to reach 364 total and edited his Hp's after calculating Stealth Rock damage WITH the resist, which is 6.25% or 22 Hp.
Modest in Rain with Life Orb
Rocks: 72.7% - 86.1%
Max Hp:68.2% - 80.7%
Modest Expert Belt:
Rocks: 66.7% - 80%
Max Hp:62.5% - 75%
Timid Expert Belt:
Rocks: 60.9% - 72.7%
Max Hp: 57.1% - 68.2%
Specially defense Ferrothron isn't a bad option with Thundurs T and Tornadus T running around, so for the sake of scenario and making a nice Theorymon. Ferrothron could still be granted the opportunity to thunder wave Hydreiogon or Gyro Ball it. Now if it is taking a hit from Hydreigon on the switch that isn't lowering Hydreigon's Sp Atk then Ferrothorn would get 2HKO'd probably unless there is a double protect or a miss.
Moreover I predict Mix Hydreigon to be very powerful and feared in the new B2W2 meta-game, because now he doesn't have to switch out against Tyranitar or Blissey. His power and versatility will have a large boost and his importance as a water resist will be more necessary than before, because I'm sure rain is about to be the most favored weather.