Hydreigon was pretty good until Fairy types were introduced. Then, it moved down a tier. It's really outclassed by other psuedo legendaries. (especially ones with mega evolution) It doesn't even get Nasty Plot. Though I still like this thing, so I gave two sets.
Main Lead
Hydreigon @ Choice Scarf
Timid Nature
Ability: Levitate
EVs: 252 Sp. Atk / 252 Spe / 4 HP
- Draco Meteor
- Dark Pulse
- Flash Cannon/U-Turn
- Focus Blast/Fire Blast
The Scarf on Hydreigon allows it to outspeed most things in UU with the exception of Mega-Aerodactyl, Mega-Beedrill, and other Scarfers. Draco Meteor is for your main STAB and is really powerful. Just switch out after the Sp. Atk drop. Dark Pulse is for STAB. Flash Cannon is if you dare to take on Fairy types, but U-Turn is for switching out against a Fairy type Pokemon. Focus Blast is better Ice coverage and Steel coverage, but Fire Blast is more accurate and does the same thing as Focus Blast with the plus of Bug coverage.
Life Orb Attacker
Hydreigon @ Life Orb
Timid Nature
Ability: Levitate
EVs: 252 Sp. Atk / 252 Spe / 4 HP
- Dragon Pulse
- Dark Pulse
- Fire Blast
- Defog/U-Turn
You really only need three moves with this set. Dragon Pulse is pretty good STAB. Dark Pulse is also good STAB. Fire Blast covers Ice, Bug, and Steel types. Defog is just for utility, but U-Turn is for scouting.