Hydreigon is very good with his 92/90/125 defences and 105/125 offences and usable even without dark typing.
This is set for all-out attacker:
- Flamethrower/Fire Blast
- Earthquake/Rock Slide
- Hyper Beam
Hydreigon has more special bulk than Dragonite/Salamence/Garchomp and is not so scared of ice-types. 3-headed dragon has movepool larger than Kingdra has.
Flamethrower and Fire blast work well against Ice-types.
Surf is needed against Rhyperior and other walls that are weak to water.
EQ is needed against special walls such as Ninetales. Rock slide is an option for outclassing Gyarados.
Hyper Beam... Well, it's Hyper Beam.