Submitting
Regenerator Nihilego as a counter.
Nihilego @ Assault Vest
Ability: Regenerator
EVs: 248 HP / 8 SpA / 252 SpD
Calm Nature
- Power Gem
- Clear Smog
- Knock Off
- Sludge Bomb
Regenvest Nihilego can tank anything Blacephalon throws at it besides Psyshock and avoid the 2hko (Psyshock OHKOs) while it still has its vest. Without its vest the set listed still can't 2hko it and dies in one hit back, although Hidden Power Ground becomes a 2hko.
0 SpA Nihilego Power Gem vs. 0 HP / 4 SpD Blacephalon: 254-302 (102.8 - 122.2%) -- guaranteed OHKO
252 SpA Life Orb Blacephalon Mind Blown vs. 252 HP / 252+ SpD Assault Vest Nihilego: 70-83 (16.5 - 19.6%) -- possible 6HKO
252 SpA Life Orb Blacephalon Shadow Ball vs. 252 HP / 252+ SpD Assault Vest Nihilego: 75-91 (17.7 - 21.5%) -- possible 5HKO
252 SpA Life Orb Blacephalon Mind Blown vs. 252 HP / 252+ SpD Nihilego: 105-125 (24.8 - 29.6%) -- 100% chance to 4HKO
252 SpA Life Orb Blacephalon Shadow Ball vs. 252 HP / 252+ SpD Nihilego: 113-134 (26.7 - 31.7%) -- guaranteed 4HKO
252 SpA Life Orb Blacephalon Hidden Power Ground vs. 252 HP / 252+ SpD Assault Vest Nihilego: 151-182 (35.7 - 43.1%) -- guaranteed 3HKO
252 SpA Life Orb Blacephalon Hidden Power Ground vs. 252 HP / 252+ SpD Nihilego: 229-270 (54.2 - 63.9%) -- guaranteed 2HKO
Just a little warning, I'm not positive I got the set quite right, so feel free to correct me on what Regenerator Nihilego usually runs. Either way, it still counters this Blacephalon set.