Aegislash @ Life Orb/Leftovers
Trait: Stance Change
EVs: 252 HP / 252 Atk / 4 SpD
- Iron Head
- King's Shield
- Sacred Sword
- Shadow Claw
The important move here is Aegislash's Iron Head. You can change Shadow Claw and stuff for Shadow Sneak if you want. Why you ask?
252+ Atk Life Orb Aegislash-Blade Iron Head vs. 252 HP / 0 Def Xerneas: 437-515 (95.8 - 112.9%) -- 68.8% chance to OHKO
252+ Atk Life Orb Aegislash-Blade Iron Head vs. 4 HP / 0 Def Xerneas: 437-515 (110.9 - 130.7%) -- guaranteed OHKO
Unlike a lot of the previous answers, Aegislash outright OHKOs most Xerneas WITHOUT boosts
Even more importantly
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. 252 HP / 0 SpD Aegislash-Shield: 161-190 (49.6 - 58.6%)
+2 252+ SpA Xerneas Hidden Power Fire vs. 252 HP / 0 SpD Aegislash-Shield: 204-242 (62.9 - 74.6%)
None of Xerneas's moves will be able to OHKO Aegislash even at +2, and at best 3HKO without any boosts
This means Aegislash can switch into Xerneas with very little fear, provided it has not already got up the Geomancy - because it can easily take any unboosted move Xerneas throws at it, as well as tank a +2 if Xerneas decides to set up Geomancy on the turn you switch in.
+2 252+ SpA Fairy Aura Xerneas Moonblast vs. 4 HP / 0 SpD Talonflame: 310-366 (104 - 122.8%) -- guaranteed OHKO
+2 252+ SpA Xerneas Hidden Power Fire vs. 252 HP / 4 SpD Mega Scizor: 576-680 (167.4 - 197.6%) -- guaranteed OHKO
252+ SpA Fairy Aura Xerneas Moonblast vs. 0 HP / 4 SpD Mega Lucario: 306-361 (108.8 - 128.4%) -- guaranteed OHKO
The other listed counters can still be worked around by Xerneas - Aegislash on the other hand, checks it completely, provided it doesn't try switch into a +2 HP Fire.