I'd advise that you run Scarf.
It's helped me in the past, and I'm sure it'll help you.
Here's what I run on my Hydreigon:
252 Spd | 252 SpAtk | 4 HP
Modest Nature since Scarf normally helps boost Speed, you can run Timid if necessary.
- Fire Blast / Flamethrower, a matter of power over accuracy.
- Draco Meteor / Dragon Pulse
- U-turn / Focus Blast
You can't take out a Sylveon at all, nor can you hit mGarde hard enough. You can't take out any Fairies at all. Pull out at all costs.
252 SpA Pixilate Mega Gardevoir Hyper Voice vs. 0 HP / 4 SpD Hydreigon: 996-1176 (306.4 - 361.8%) -- guaranteed OHKO
4 SpA Togekiss Dazzling Gleam vs. 0 HP / 4 SpD Hydreigon: 436-520 (134.1 - 160%) -- guaranteed OHKO
4 SpA Pixilate Sylveon Hyper Voice vs. 0 HP / 4 SpD Hydreigon: 600-708 (184.6 - 217.8%) -- guaranteed OHKO
252+ Atk Slurpuff Play Rough vs. 0 HP / 0 Def Hydreigon: 508-604 (156.3 - 185.8%) -- guaranteed OHKO
252+ Atk Huge Power Azumarill Play Rough vs. 0 HP / 0 Def Hydreigon: 780-924 (240 - 284.3%) -- guaranteed OHKO
252 SpA Alakazam Dazzling Gleam vs. 0 HP / 4 SpD Hydreigon: 392-464 (120.6 - 142.7%) -- guaranteed OHKO
(But Hydreigon outspeeds Alakazam and can go for the kill if it has Scarf.)
252 SpA Hydreigon Dark Pulse vs. 4 HP / 0 SpD Alakazam: 266-314 (105.5 - 124.6%) -- guaranteed OHKO
Moral of the story, don't leave Hydreigon out against Fairy-type move users. And don't be tempted to use Flash Cannon at all. It's pretty much useless when the best you can do is 2HKO a Fairy, in which YOU'RE ALREADY DEAD.
Anyway, I'd like to say I just got a shiny Hoppip from Horde RE! YAY!!!!
EDIT: Dragonite can take a few hits, and if you give it a Weakness Policy, it becomes your worst nightmare. The only things that can touch it are mGarde and mWile, which is pretty bad. However, with hazards up, DNite will suffer due to a lack of Multiscale. If you decide to run a DNite, you need a Spinner / Defogger.
Explosion on Genesect is a big gamble, as most things die to it, but common mons like Heatran and the myriad Ghosts that haunt OU just say no.