Mamoswine is an incredibly powerful Pokemon, having an impressive 130 Attack Stat. And backed up by the 100 Power move Earthquake with the 1.5 power up from STAB, it is a monster to face. And many people tend to use the Item Choice Band, boosting the already insane Atttack by 1.5. Very few Pokemon can take a hit from it, even if it is a neutral attack. The case might also be that your opponents EV train their Pokemon and you don't. EV trained Pokemon are many times over more powerful than their untrained counterparts.