Serebii is still using some outdated info on their stage %'s, so watch out on that. It also still lists focus energy as a 1 stage boost (it's 2).
While the Nido's were a bad idea, I'll defend focus energy Hydreigon sets.
Hydreigon @ Scope Lens
Ability: Levitate
Nature: Naive/mild
EV's: 252 SpA/ 252 Spe/ 4 Atk
-Focus Energy
-Draco Meteor
-Superpower
-Fire Blast/Flash Cannon/Charge Beam/Dark Pulse
His only boosting move is work up but that's no good if your main moves lower your stats!
Now you can spam choice-specs Draco Meteors and Superpowers all day long, but without the choice lock. This would be better if he actually got a poison type move, but Fire Blast for Mawhile and Ferrothorn, Flash Cannon for Sylveon/Gardevoir, Charge Beam for Azumarill and Togekiss... maybe? This really depends on him 2KOing each of them respectfully, one on the switch in and the second to finish them off since none of them outspeed and he resists sucker punch and aqua jet.
Can somebody run the calcs on that? Also see if Mawhile can take a superpower before and after mega evolving. Remember that intimidate is ignored due to the critical hit.
Unfortunately, Mawile can take a Superpower even without Mega Evolving, and even more so after Mega Evolving, however, even Flamethrower is a clean OHKO
+1 0 Atk Hydreigon Superpower vs. 252 HP / 0 Def Mawile: 154-182 (50.65 - 59.86%) -- guaranteed 2HKO
+1 0 Atk Hydreigon Superpower vs. 252 HP / 0 Def Mega-Mawile: 112-132 (36.84 - 43.42%) -- guaranteed 3HKO
+1 252+ SpA Hydreigon Flamethrower vs. 252 HP / 0 SpD Mega-Mawile: 328-388 (107.89 - 127.63%) -- guaranteed OHKO
Charge Beam is still very weak, especially against Togekiss and Azumarill which are already quite tanky
+1 252+ SpA Hydreigon Charge Beam vs. 212 HP / 0 SpD Azumarill: 212-250 (53.8 - 63.45%) -- guaranteed 2HKO
+1 252+ SpA Hydreigon Charge Beam vs. 248 HP / 0 SpD Togekiss: 156-184 (41.82 - 49.32%) -- guaranteed 3HKO
Flash Cannon too, is hard to get an OHKO
+1 252+ SpA Hydreigon Flash Cannon vs. 252 HP / 252+ SpD Gardevoir: 182-216 (53.52 - 63.52%) -- guaranteed 2HKO
+1 252+ SpA Hydreigon Flash Cannon vs. 252 HP / 4 SpD Gardevoir: 248-292 (72.94 - 85.88%) -- guaranteed 2HKO
+1 252+ SpA Hydreigon Flash Cannon vs. 252 HP / 4 SpD Sylveon: 222-262 (56.34 - 66.49%) -- guaranteed 2HKO
At the end of the day, Hydreigon is not getting past any Fairies minus Mawile anytime soon, unless you nail them on the switch, which only Togekiss survives the 2HKO, unless you get a lucky Charge Beam +1.