I was going to nominate Heatmor as a counter but then realised that I forgot to put sheer force. Dammit.
But anyway Heatmor is a really good offensive check. I run 136 speed to outspeed adamant mawile by one point, so that it is forced to click sucker punch or else get oneshot without doing any damage (play rough actually does more believe it or not). Heatmor lives sucker punch even after SD relatively well and as I may have implied above, I have run enough attack on the set to oneshot with fire lash if the player is scared that they miss with fire blast. I'm not going to include +4 calcs because firstly, they OHKO everything except klang or something, and nobody should be allowing an opponent mawile to get to +4. I'm going to be listing calcs in the order: Best for
--> best for
.
Living
Sucker Punch
+2 252+ Atk Life Orb Mawile Sucker Punch vs. 232 HP / 4 Def Heatmor: 227-269 (61.5 - 72.8%) -- guaranteed 2HKO
252+ Atk Life Orb Mawile Sucker Punch vs. 232 HP / 4 Def Heatmor: 114-135 (30.8 - 36.5%) -- 64% chance to 3HKO
Oneshotting
Defensive Set: 136+ Atk Heatmor Fire Lash vs. 0 HP / 4 Def Mawile: 242-288 (100.4 - 119.5%) -- guaranteed OHKO
Defensive Set: 0 SpA Heatmor Fire Blast vs. 0 HP / 0 SpD Mawile: 398-470 (165.1 - 195%) -- guaranteed OHKO