EV
Banned deucer.
Ferrothorn is a better Grass-type, but the neutrality to EQ lets it down, especially since Spiky/King's Shield won't activate their effects from it due to no contact.
252+ Atk Huge Power Diggersby Earthquake vs. 252 HP / 252+ Def Ferrothorn: 127-150 (36 - 42.6%) -- 94.9% chance to 3HKO after Leftovers recovery
+2 252+ Atk Huge Power Diggersby Earthquake vs. 252 HP / 252+ Def Ferrothorn: 252-297 (71.5 - 84.3%) -- guaranteed 2HKO after Leftovers recovery
252+ Atk Huge Power Diggersby Earthquake vs. 252 HP / 252+ Def Ferrothorn: 127-150 (36 - 42.6%) -- 94.9% chance to 3HKO after Leftovers recovery
+2 252+ Atk Huge Power Diggersby Earthquake vs. 252 HP / 252+ Def Ferrothorn: 252-297 (71.5 - 84.3%) -- guaranteed 2HKO after Leftovers recovery