Yes, Hydreigon is a very powerful special attacker, although it has often been used as a mixed attacker. There are many good options for it in-game including, Dragon pulse, Surf, Fire Blast, U-Turn, Draco Meteor, Focus Blast, Earth Power, Flash Cannon and Dark Pulse.
For an in-game set I would suggest,
Hydreigon @ Life Orb (or whatever doesn't matter much)
EVs (if you EV train): 252 SAtk / 252 Spd / 4 HP
- Draco Meteor/Dragon Pulse
- Earth Power
- Fire Blast
- Surf/Dark Pulse
Go for Surf if you don't have another Pokemon who can use Surf. Fire Blast and Earth Power make great coverage, while Draco Meteor and Pulse are your choices of STAB with the first having a drawback (lowers special attack) but more power than the second.