Head smash could be a viable option for fairies since it has more power than steel wing and slighty better accuracy than iron tail, plus it gives him better coverage.
0 Atk Life Orb Hydreigon Head Smash vs. 252 HP / 0 Def Azumarill: 177-208 (43.8 - 51.4%) -- 8.2% chance to 2HKO guaranteed after stealth rock.
0 Atk Life Orb Hydreigon Head Smash vs. 252 HP / 0 Def Togekiss: 307-361 (82 - 96.5%) -- guaranteed 2HKO after Leftovers recovery some EV´s can be moved to attack to OHKO.
0 Atk Life Orb Hydreigon Head Smash vs. 252 HP / 0 Def Sylveon: 207-244 (52.5 - 61.9%) -- 98.8% chance to 2HKO after Leftovers recovery.
0 Atk Life Orb Hydreigon Head Smash vs. 252 HP / 0 Def Clefable: 190-224 (48.2 - 56.8%) -- 36.3% chance to 2HKO after Leftovers recovery
0 Atk Life Orb Hydreigon Head Smash vs. 252 HP / 0 Def Mega Gardevoir: 207-244 (60.8 - 71.7%) -- guaranteed 2HKO
Also unlike Iron tail or Charge beam Head smash 2HKO all the fairies that pose a problem for him, except for carbink maybe but that thing doesn´t even OHKO Hydreigon with Moonblast
0 Atk Life Orb Hydreigon Head Smash vs. 252 HP / 0 Def Azumarill: 177-208 (43.8 - 51.4%) -- 8.2% chance to 2HKO guaranteed after stealth rock.
0 Atk Life Orb Hydreigon Head Smash vs. 252 HP / 0 Def Togekiss: 307-361 (82 - 96.5%) -- guaranteed 2HKO after Leftovers recovery some EV´s can be moved to attack to OHKO.
0 Atk Life Orb Hydreigon Head Smash vs. 252 HP / 0 Def Sylveon: 207-244 (52.5 - 61.9%) -- 98.8% chance to 2HKO after Leftovers recovery.
0 Atk Life Orb Hydreigon Head Smash vs. 252 HP / 0 Def Clefable: 190-224 (48.2 - 56.8%) -- 36.3% chance to 2HKO after Leftovers recovery
0 Atk Life Orb Hydreigon Head Smash vs. 252 HP / 0 Def Mega Gardevoir: 207-244 (60.8 - 71.7%) -- guaranteed 2HKO
Also unlike Iron tail or Charge beam Head smash 2HKO all the fairies that pose a problem for him, except for carbink maybe but that thing doesn´t even OHKO Hydreigon with Moonblast