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%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.
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.