Been trying to catch up on this thread since it opened, and my god you guys work quickly. Not sure if it's already been posted, since I've been reading every other page today.
Did some damage calcs on the Lvl 50. Hydreigon vs. Gardevoir clip and Fairy Wind. Super effectiveness is at 2x, since if it were 4x the move would have to have 40 or less BP. If it were neutral, it would be very powerful. One can thus presume that Fairy is neutral against Dark.
0 SpA Gardevoir Fairy Wind (BP 95) vs. 0 HP / 0 SpD Hydreigon: 96-114 (57.48 - 68.26%) -- guaranteed 2HKO
0 SpA Gardevoir Fairy Wind (BP 80) vs. 0 HP / 0 SpD Hydreigon: 80-96 (47.9 - 57.48%) -- 90.23% chance to 2HKO
252 SpA Gardevoir Fairy Wind (BP 78) vs. 4 HP / 0 SpD Hydreigon: 96-114 (57.14 - 67.85%) -- guaranteed 2HKO
252 SpA Gardevoir Fairy Wind (BP 65) vs. 4 HP / 0 SpD Hydreigon: 80-96 (47.61 - 57.14%) -- 90.23% chance to 2HKO
I think that the most likely circumstances of that battle don't include optimal Smogon sets. I would place the base power somewhere around 90. A solid move, likely to be a staple among fairy types.
PSEUDO-EDIT: Ohhh shit, forgot STAB. Probably much weaker, or that was a very defensive Hydreigon.
I love the new Pokemon so far, though Skrelp seems a little busy to me. We even got that bullet shrimp we've been hoping for!