I know Wishbliss can outstall Scarfchomp's Outrages, so I wouldn't put much faith on Hydreigon's...
Life Orb...
Mixed Hydreigon should be running Outrage / Crunch / Fire Blast / Draco Meteor with Lonely and 252 Atk / 4 SpA / 252 Spe.
Crunch vs:
252/252+ Blissey: 40.9% - 48.3%
252/252+ Reuniclus: 65.6% - 77.8%
252/252+ Jellicent: 71.3% - 84.7%
252/120 Tentacruel: 53.6% - 62.9%
252/252+ Vaporeon: 33.8% - 40.1%
252/152+ Hippowdon: 26.4% - 31.4%
Outrage vs:
252/252+ Blissey: 61.1% - 72%
252/252+ Reuniclus: 49.5% - 58.3%
252/252+ Jellicent: 53.7% - 63.4%
252/120 Tentacruel: 79.4% - 94%
252/252+ Vaporeon: 51.1% - 60.3%
252/152+ Hippowdon: 40% - 47.1%
Fire Blast vs:
252/252+ Skarmory: 74.3% - 87.4%
252/160+ Nattorei: 109.1% - 128.4% (OHKO)
240/76 Jirachi: 62.8% - 74.3%
252/104 Hippowdon: 37.1% - 43.8%
Draco Meteor vs:
252/0 Gliscor: 85% - 100.3%
252/104 Hippowdon: 64.5% - 76.4%
252/4 Vaporeon: 53.2% - 62.9%
252/4 Blissey: 25.8% - 30.4%
- Blissey is 2HKOed by Crunch + Outrage. Chance of 2HKO with Draco Meteor + Outrage.
- Reuniclus is 2HKOed by any move (including Fire Blast) + Crunch
- Hippowdon is 2HKOed by Fire Blast + Draco Meteor, or Draco Meteor + Outrage.
- Gliscor is 2HKOed by any move + Draco Meteor. Chance of OHKO with Draco Meteor.
- Vaporeon is 2HKOed by Crunch + Draco Meteor with Stealth Rock, or Draco Meteor + Outrage.
- Reuniclus is 2HKOed by Fire Blast/Crunch + Crunch/Draco Meteor, or Crunch + Outrage.
- Jellicent is 2HKOed by Draco Meteor/Crunch + Outrage/Crunch.
- Tentacruel is 2HKOed by Crunch + Crunch/Outrage, or Draco Meteor + Outrage.
- Skarmory is 2HKOed by Fire Blast + Outrage/Crunch/Fire Blast.
- Nattorei is OHKOed by Fire Blast.
- Jirachi is 2HKOed by Crunch/Fire Blast + Fire Blast.
Hydreigon 2HKOs a lot of walls and tanks, as you can see. It has the advantage of not being forced into Outrage in order to hit stuff hard on the physical side, unlike Salamence. There's probably other calculations I could run, but I think those are the most important defensive threats. It's really effective.