For those struggling with setup in this meta:
Clefable @ Leftovers
Ability: Unaware
EVs: 252 HP / 252 Def / 4 SpA
Bold Nature
- Wish
- Protect
- Thunder Wave/Stealth Rock
- Moonblast
This is an solid set in this meta for Fairy teams (so Xerneas, basically). You can take any one hit from Mega Rayquaza even if its set up and get off a Thunder Wave or Moonblast (only kills if it's ~60%, because sadly Unaware also ignores the drops from Dragon Ascent). But more directly, it shuts down Arceus:
252 Atk Life Orb Arceus Extreme Speed vs. 248 HP / 252+ Def Clefable: 142-168 (36.1 - 42.7%) -- guaranteed 3HKO after Stealth Rock and Leftovers recovery
It also helps for Normal teams as a whole, as Meloetta and Mega Pidgeot are really your only options when it comes to Special Attackers, so you can stall out many threats with Wishtect.
Here's a replay of me doing just that.
You can also go Specially Defensive as a secondary check to other Xerneas (all Xern teams should have Klefki, but if it gets weakened, this ensures you don't lose):
252+ SpA Fairy Aura Xerneas Moonblast vs. 248 HP / 252+ SpD Clefable: 177-208 (45 - 52.9%) -- guaranteed 3HKO after Leftovers recovery
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On an unrelated note, has anyone been using/seeing Mega Gengar? I think trapping could be very valuable here -- Mega Rayquaza, for instance, can be killed after it Dragon Ascents if Mega Ray has either taken one additional Life Orb hit or a Stealth Rock switch-in.
252 SpA Mega Gengar Sludge Wave vs. -1 0 HP / 0 SpD Mega Rayquaza: 286-337 (81.4 - 96%) -- guaranteed 2HKO
Your E-speed immunity also comes in handy here. Other, frailer gods like Shaymin-S can simply be outspeed and OHKO'd. Sub + Perish Song + Protect has promise as well for bulky gods, although many of them carry Roar which ruins this strategy.