There's not a lot of fast Fairy type Pokemon, and we're definitely not getting any Legendaries by the time we fight her. Game mechanic wise, let's assume that Ame's A-Ninetales is Modest Nature. It couldn't OHKO Hydregion, and the only way it would've lived is if it ran a SpDef Nature as well as perfect IVs and EVs on its HP and SpDef. Unless A-Ninetales also had Choice Specs or Life Orb(or any item that will boost Moonblast). Her Hydregion is fucking hacked.
I should also mention that they were on a Factory Field, and as far as I know, there's no downside for Fairy moves.