Mamoswine is a Pokemon with many great traits, and many serious flaws; starting with the flaws, he has 5 weaknesses: Fire, Water, Grass, Steel, and Fighting, most of which are easily exploitable. This, combined with one useless resistance to Poison, one immunity to Electric that is difficult to take advantage of, and subpar Speed and defenses, greatly hamper Mamoswine's potential.
However, it's in the lead position, where these flaws are less significant, that his traits truly stand out. Stealth Rock, a fairly powerful STAB Earthquake, STAB priority that destroys Shaymin-S, a demon to most suicide leads, and the ability to retain his Focus Sash in both sandstorm and hail, combine together to make Mamoswine a surprisingly effective suicide lead.
Mamoswine can set Stealth Rock up against every common lead bar Darkrai. Earthquake is his main STAB, whilst Ice Shard finishes off weakened foes, particularly ones who survive on 1 HP thanks to Focus Sash, and destroys Shaymin-S, who troubles most other suicide leads. After setting up Stealth Rock, Mamoswine should use Endeavour against slower leads, like Dialga and Kyogre, to bring their HP down, often to 1 HP thanks to Mamoswine’s subpar Special Defense and exploitable weaknesses. These foes can then be easily finished off by a teammate.Team Options & Additional Comments >>>
Avalanche is an option over Endeavor to deal more damage than Earthquake if Mamoswine takes a hit. It almost always OHKOes Giratina-O (it has a small chance to survive if it isn't running a Defense reducing nature). Even if it does survive, Ice Shard will finish it off before it can Shadow Sneak unless it has invested heavily in Speed. Avalanche + Ice Shard usually beats Careful Groudon, unfortunately, this nature is not always used, and this combination will fail against an Impish Groudon.
Mamoswine can be used outside the lead role for a more offensive approach, wielding a Choice Band or Life Orb, but Mamoswine lacks the Speed to be an effective sweeper or wallbreaker, and its only high-powered STAB without a drawbrack, Earthquake, doesn't affect many of the Ubers. Also, being locked into a move which something like Rayquaza can come into and get a free turn of set up is not a good idea. STAB Ice Shard generally does not compensate for Mamoswine's faults. The former is very predictable, and the only foe who fears it and who Mamoswine can reliably switch into is Garchomp, whom Mamoswine can only switch into once, as Mamoswine gets 2HKOed by Earthquake. Rayquaza, Latias, and Latios can all OHKO Mamoswine on the switch-in with the right attack, even Latias's Dragon Pulse can OHKO him with Stealth Rock support. Otherwise Ice Shard usually doesn't deal enough damage to Pokemon that aren't weak to it. Avalanche may initially be more appealing on such a set, but Mamoswine cannot afford to move last; his common weaknesses and poor Defenses mean many attacks in Ubers will OHKO him.
Checks and Counters
Darkrai puts Mamoswine to sleep with Dark Void while simultaneously removing his Focus Sash with Bad Dreams. Mamoswine only removes, on average, less than a third of Darkrai's HP with Ice Shard. Deoxys-A can set up Stealth Rock, bring Mamoswine down to 1 HP, and then finish him off with ExtremeSpeed before he can do the same with Ice Shard. Lead Rayquaza holding a Focus Sash will either let Mamoswine set up Stealth Rock while it remains at full HP, or Mamoswine will break its Sash and be KOed in the process, all the while not being able to set up Stealth Rock due to Rayquaza's ExtremeSpeed. Forretress removes Stealth Rock and Mamoswine's Focus Sash with Rapid Spin whilst setting up its own entry hazards and only being 3HKOed or 4HKOed by Earthquake depending on the damage rolls.