Answer 1

Steel types, like Scizor. Scizor is a great counter because it's not weak to Ground type attacks, and Ice type moves are not very effect against it.

Scizor @ Life Orb

Trait: Technician
EVs: 252 HP / 252 Atk / 4 Spe
Adamant Nature (+Atk, -SpA)

• Swords Dance
• Bullet Punch
• Bug-Bite
• Roost/Superpower

Swords Dance is needed, and Mamoswine can't OHKO Scizor, giving Scizor a chance to set up. Then use Bullet Punch to hopefully KO the Mamoswine.

Comments

Thanks everybody I can't choose which one is best because they are all great answers.

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Usually you pick the one that came in first, and up vote all the rest.

Answer 2

If you are asking for a good counter for mamoswine then read this:

Steel-types can be tough on Mamoswine, hitting it for super effective damage and having the ability to come in on one of its STAB moves. Scizor smashes Mamoswine with Bullet Punch, while Jirachi nails it with Iron Head. Gyarados can set up on Mamoswine using the Attack drop from Intimidate to force Mamoswine out, allowing it to set up a Dragon Dance. Any Pokemon with above average Defense and defensive typing should, in theory, be able to stop Mamoswine in its tracks if it is locked into a move with Choice Band or Choice Scarf. Ferrothorn and Bronzong are excellent examples of this, both having immense base Defense and the ability to KO Mamoswine with their STAB moves. Water-types are also great counters to Mamoswine, as they resist its Ice-type attacks and hit it for super effective damage. - Smogon

Comments

But steel is weak to ground :O

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Scizor is part bug, Jirachi is faster in most cases, ferrothorn is part grass, and bronzong usually has levitate. so only one of those steels is weak to ground. And you may be a better battler at me in the higher OU/UU, the writers at smogon know what they're doing, so if you wanna be the one to question them, go right ahead.

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Giga, you just got schooled! xD

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---O.O---

Answer 3

A physically defensive water type is nice. Water types resist mamoswine's ice type moves, and in turn get STAB water type attacks that are Super Effective on Mamoswine. Some physically defensive water types would be...
-Suicune
-Slowbro
-Feraligatr
-Kingler
-Cloyster
-Azumarill
-Kingdra
-Huntail
-Walrein
-Poliwrath

Answer 4

I'd suggest you use a fire-flying type, like Moltres because if Mamoswine attempts to use a ground type attack, it wouldn't affect a flying type, plus, with fire type moves like fire blast of fire spin etc, you would wipe the floor with Mamoswine. If you don't have a Moltres, I'd recommend Ho-oh, it's Sacred Fire attack would take Mamoswine to school alright! Or maybe a water type like Feraligatr, Blastoise etc, because water beats ground and it has a normal effect on ice and it's super effective on ground. Plus, both ground and ice type moves have a normal effect on water type pokemon. Or steel type pokemon would do too.