Arceus-Rock @ Stone Plate
Ability: Multitype
EVs: 252 HP / 240 Def / 16 Spe
Bold Nature
IVs: 0 Atk
- Judgment
- Earth Power
- Calm Mind / Will-O-Wisp
- Recover
Arceus-Rock is one of the most reliable Pins Mage answers, and is a solid check to other tough metagame threats like Pinsirite Entei. Will-O-Wisp, while not necessary to check Pins Mage, is a helpful option that can do a ton as it can hinder Magearna's Attack (though it's generally best to just spam Judgment so there's no mindgames involved with Substitute).
Defog or Stealth Rock can be replaced by any filler move really such as Refresh, Earth Power or even Calm Mind. EDIT: Chazm reminded me that pre-mega Magearna can sub up against mono-Judgment attacking Rockceus, so Earth Power is the only 4th move I'd recommend in this situation.
Calcs:
+1 252+ Atk Aerilate Magearna Return vs. 252 HP / 240+ Def Arceus-Rock: 102-120 (22.9 - 27%) -- 52% chance to 4HKO
+2 252+ Atk Aerilate Magearna Return vs. 252 HP / 240+ Def Arceus-Rock: 135-160 (30.4 - 36%) -- 49.3% chance to 3HKO
0 SpA Stone Plate Arceus-Rock Judgment vs. 36 HP / 0 SpD Magearna: 117-138 (37.7 - 44.5%) -- guaranteed 3HKO