Smogon: Due to its high Attack and powerful STAB moves, Mamoswine has few counters. Skarmory and Bronzong are its best counters, taking relatively little damage from its attacks and threatening to set up entry hazards or KO it. Rotom-W also switches into most of Mamoswine's moveset, but takes substantial damage from Life Orb Superpower. It is also important to note that all of these switch-ins risk having their HP greatly lowered by Endeavor when switching in on Focus Sash Mamoswine. It is significantly easier to check Mamoswine than to counter it, as Mamoswine's Ice / Ground typing and middling Speed makes it vulnerable to super effective Fighting- and Water-type attacks. Pokemon that can outspeed Mamoswine, withstand Ice Shard, and hit it super effectively, such as Keldeo, Terrakion, and Scizor, all solidly check Mamoswine.
.