There are better choices for a Hydreigon set. I could probably tailor the EVs if I knew what the set is trying to do. Is Earthquake for Heatran? If you want to run a physical set, Replace Dragon Pulse with either Dragon Claw or Outrage, and give it the following EVs: 252 Attack / 4 Special Attack / 252 Speed with a Lonely or Naughty nature. If you switch Flamethrower to Fire Blast and give Hydreigon a Life Orb, you have a guaranteed OHKO on all variants of Ferrothorn. Heatran and Skarmory are OHKOed by Earthquake and Fire Blast, respectively.
The set is:
Hydreigon @ Life Orb
Lonely / Naughty (Naughty is preferable)
252 Atk / 4 SpA / 252 Spe
Outrage / Dragon Claw
Crunch
Earthquake
Fire Blast