Overview
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Mamoswine stands in the OU metagame as a powerful offensive threat thanks to its great Attack stat and STAB coverage of Ice and Ground, which is only resisted by Rotom (specify which formes you meant, because Rotom itself is Electric/Ghost and doesn't resist Ice) and Bronzong. Its Ground typing also grants Mamoswine it an immunity to Electric, thereby allowing it to check threats such as Thundurus and Raikou. Mamoswine has two great abilities that allow it to either remove its Fire weakness and add an Ice resistance with Thick Fat or prevent the effects of Taunt, giving to give it utility as a Focus Sash Lead that is able to reliably set up Stealth Rock. In addition, its access to priority Ice Shard gives it a way to bypass to its mediocre base 80 Speed stat, and Ice Shard is useful for as well as revenge killing Dragon-types such as Mega Sceptile, Latios, Latias, and Altaria, (remove comma) and Flying-types such as Thundurus and Mega Aerodactyl.
Unfortunately, bulky Water-types including Slowbro and Alomomola often get free switches into switch-ins to Mamoswine and proceed to wall it even though this is somewhat remedied by Freeze-Dry. Mamoswine also lacks strong defensive stats and many resistances, making it difficult for it to live survive more than one powerful attack. Its mild slowness is its downfall when facing fast threats that are able to tank Ice Shard such as Mega Metagross and Diancie. Other physically defensive walls such as Ferrothorn and Skarmory also check Mamoswine easily. However, Mamowsine has a strong niche thanks to its unique STAB combination moves, strong physical attacking Attack stat, and access to Stealth Rock.
Life Orb
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name: Life Orb
move 1: Ice Shard
move 2: Earthquake
move 3: Icicle Crash / Icicle Spear
move 4: Freeze-Dry
ability: Thick Fat
item: Life Orb
evs: 244 Atk / 12 SpA / 252 Spe
ivs: 29 HP
nature: Naive
Moves
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Ice Shard is mandatory on Mamoswine to allow it to revenge kill Dragon- and Flying-types such as Landorus, Mega Altaria, Landorus, and Thundurus. Earthquake is a powerful STAB move. Icicle Crash is a secondary STAB move that has near-perfect coverage with Earthquake and a nice flinch chance, but it is inaccurate. Icicle Spear can be used in place of Icicle Crash for the same perfect (you said Icicle Crash is near-perfect, so Icicle Spear can't be perfect) coverage and the ability to anti-lead Breloom and hit through Dragonite's Multiscale. Freeze-Dry is important despite Mamoswine's low Special Attack because it hits bulky Water-types such as Rotom-W and Alomomola. Other moves that can be used include Knock Off, which removes Skarmory's Shed Shell; Superpower, which 2HKOes Ferrothorm; and Stealth Rock in case you lack another Stealth Rock user.
Set Details
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Life Orb gives Mamoswine great amount of both physical and special power and allows it to OHKO offensive Garchomp with Ice Shard. 244 Attack EVs take advantage of its base 130 Attack stat. The Special Attack investment allows Mamoswine to 2HKO bulky Water-types such as Rotom-W and Slowbro with Freeze-Dry. Maximum Speed and a Naive nature lets Mamoswine Speed tie with Gallade before it Mega Evolves and outspeed Breloom and Dragonite. 29 HP IVs reduce Life Orb recoil by 1 point. Thick Fat is helpful to tank occasional Fire- or Ice-type moves thrown at Mamoswine.
Usage Tips
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Mamowsine can revenge kill threats such as Mega Sceptile, Garchomp, and Altaria (if weakened), so it's nice to keep alive around for late-game. Mamoswine's Ice-type STAB moves pressures Stealth Rock setters and entry hazard removers such as Landorus and Latios, (comma) respectively. This can be abused to prevent entry hazards from being set up or to keep them on the foe's side. Bulky Water-types are common checks to Mamoswine, so it's easy to predict them by using Freeze-Dry as they on the switch in to heavily dent or 2HKO them. Only switch Mamoswine into Electric-type attacks, which Mamoswine is immune to, because other hits will dent Mamoswine's mediocre defenses (attacks dent Mamoswine, not its defenses, so I'd rephrase this). Life Orb will wear Mamoswine down, so make each hit count by predicting correctly. Avoid staying in on foes it can't beat 1v1 such as Scizor and Slowbro, as the former of which can set up on Mamoswine.
Team Options
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Magnezone and Magneton are great partners, as they can trap Ferrothorn and Skarmory, which are often used to check Mamoswine. Healing Wish users such as Latias and Lopunny help restore Mamoswine's health if it has been worn down by Life Orb or crippled by status. Ferrothorn is a nice defensive partner because it can switch into Azumarill, Scizor, and Gyarados, all of which force Mamoswine out. Azumarill has good defensive synergy with Mamoswine because it resists a lot of Mamoswine's its weaknesses and also weakens Ferrothorn. (check to make sure I read those "it"s correctly) Physical Fighting-types such as Lucario and Terrakion enjoy having Slowbro and Alomomola weakened by Freeze-Dry, giving them good offensive synergy with Mamoswine. Amoonguss and Venusaur are helpful against Azumarill. Celebi can switch into the Water-types that find themselves in Mamoswine's way, such as Slowbro and Alomomola, while also being able to to pass Mamoswine Swords Dance boosts. Shuckle and Smeargle can set up Sticky Web to remedy Mamoswine's low Speed. Mamoswine fairs fares well against entry hazard removers such as Latios, Latias, and Tentacruel, which helps keep Sticky Web on the opponent's side of the field.
Sash Lead
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name: Sash Lead
move 1: Stealth Rock
move 2: Ice Shard
move 3: Earthquake
move 4: Endeavor / Icicle Spear / Knock Off
ability: Oblivious / Thick Fat
item: Focus Sash
evs: 252 Atk / 4 Def / 252 Spe
nature: Jolly
Moves
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Stealth Rock is the main component of this set, as it deters threats such as Talonflame, Pinsir, and Kyurem-B from switching in. Ice Shard is mandatory for revenge killing and getting off chip damage on faster foes. Earthquake is a powerful STAB move that dents anything that doesn't resist it. In the last slot, Endeavor is nice to bring foes down to 1 HP after Mamoswine has burned used its Focus Sash, which is useful for crippling walls. Icicle Spear is viable as well to deal with anti-lead Breloom and hurt Defog and Stealth Rock users such as Mandibuzz and Landorus-T, (comma) respectively. Knock Off is nice for removing Shed Shell from Skarmory, allowing it to be trapped by Magnet Pull. Superpower can be used to 2HKO Ferrothorn.
Set Details
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A Jolly nature and 252 Speed EVs allow Mamoswine to outspeed Breloom, Bisharp, Gallade before it Mega Evolves, and bulky Jirachi, Gallade before it Mega Evolves, and set up Stealth Rock before it gets hit. Focus Sash guarantees the chance to that Mamoswine can set up Stealth Rock by allowing Mamoswine to take any one hit. Maximum Attack investment takes advantage of Mamoswine's base 130 Attack stat and gives Ice Shard the power it needs to revenge kill. Oblivious prevents Taunt used by Azelf and Prankster Thundurus from disabling Stealth Rock. If Taunt protection is deemed unnecessary, Thick Fat may be used to tank Fire- or and Ice-type moves without burning Mamoswine's Focus Sash.
(I realize that first change looks pretty weird/unnecessary, so here's my explanation:
A Jolly nature and 252 Speed EVs allow Mamoswine to outspeed Breloom, Bisharp, bulky Jirachi, Gallade before it Mega Evolves, and set up Stealth Rock before it gets hit.
Because there's no "and" before "Gallade", the list of Pokemon that Mamoswine outspeeds does not stop at Gallade. It instead sounds like "set up Stealth Rock" is something that Mamoswine outspeeds. So, I changed it to this:
A Jolly nature and 252 Speed EVs allow Mamoswine to outspeed Breloom, Bisharp, bulky Jirachi, and Gallade before it Mega Evolves, (remove comma) and set up Stealth Rock before it gets hit.
I added the and and removed the comma because there's no subject after the comma. However, now there's "Gallade before it Mega Evolves" and "set up SR before it gets hit". These "it"s are referring to two different Pokemon, Gallade and Mamoswine. As for other solutions, "Gallade before Gallade Mega Evolves" and "set up Stealth Rock before Mamoswine gets hit" both sounded silly, as did "pre-Mega Gallade". Also, "non-Mega Gallade" kind of implied that Gallade wasn't running its Mega Stone at all. So, I just moved the Gallade example so that it's not the closest one to "set up Stealth Rock" to put some distance between the "it"s. Not really a great solution, but there you go.)
Usage Tips
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Mamoswine is useful to lead with in order to set up Stealth Rock as soon as possible. The Focus Sash is nice to save for emergency use late-game provided that entry hazards are kept off your side of the field. Don't stay in on unfavorable matchups if Stealth Rock are is already up or unnecessary on the opponent's side in order to prevent your Mamoswine's Focus Sash from being lost (or "prematurely broken" or "unnecessarily broken" or something) burned. Ice Shard is good reason to save Mamoswine for late-game because with it, Mamoswine it can revenge weakened threats such as Garchomp, Thundurus, or and Altaria. Use Endeavor is meant for using against walls or bulky foes Mamoswine can KO in combination with Ice Shard. Icicle Spear is useful when faced against facing an opposing lead, as it can hit through Focus Sash (3 hits and Ice Shard KO Azelf, while 3 hits KOes Breloom). Switch out against bad lead matchups such as against Skarmory and Rotom-W because you can set up Stealth Rock later, especially if Mamoswine's Focus Sash is intact. If Mamoswine has Knock Off, use it on Skarmory if the opportunity presents itself to remove its Shed Shell.
Team Options
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Fairy-types such as Sylveon or and Gardevoir are necessary to check Mega Sableye, which will burn Mamoswine and reflect Stealth Rock. Fast offensive teammates such as Dragonite, Talonflame, or and Lopunny enjoy having Stealth Rock to guarantee OHKOs. Magnezone and Magenton are great to trap Skarmory and prevent it from using Defog. Healing Wish users such as Jirachi, Lopunny, or and Latias are nice to restore Mamoswine's Focus Sash if it does not have it anymore. (Sash disappears after usage though; it won't reappear if Mamoswine is restored to full health like Sturdy does. I'm pretty sure?) Lopunny also enjoys having its common revenge killer, Talonflame, weakened by Stealth Rock. Azumarill, Talonflame, and Gallade love having Slowbro and other bulky Water-types crippled by Endeavor. Celebi beats the bulky Water-types that hinder Mamoswine with Nasty Plot or Perish Song.
Other Options
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Toxic is an option to beat bulky Water-types, though Freeze-Dry is superior on the offensive teams Mamoswine finds itself on. Choice Band can boost Mamoswine's power without Life Orb recoil, but it prevents Mamoswine from abusing its great coverage. Rock Slide and Stone Edge have great coverage with Earthquake, but they are inferior to Mamoswine's Ice-type STAB moves. Choice Scarf is usable for revenge killing purposes. However, Ice Shard is usually enough for this role.
Checks & Counters
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**Super Effective Priority Moves** Scizor and Azumarill both resist Ice Shard and can revenge kill Mamoswine with their super effective STAB priority moves. Conkeldurr can easily tank an Ice Shard and threaten Mamoswine with a super effective Mach Punch.
**Mega Sableye**: Physically defensive Mega Sableye with a physically defensive spread can usually tank any hit from Mamoswine, but most and, more importantly, reflect back Stealth Rock. It also has Prankster prior to Mega Evolving, which allows it to burn Mamoswine and cripple it.
**Physical Walls**: Physically defensive Water-types such as Alomomola and Slowbro can usually switch in on Mamoswine, as they resist Ice and shrug off Earthquake damage. Rotom-W is an especially good [check/counter/whatever, I'll leave it to you], as it is also immune to Earthquake and can burn with Will-O-Wisp or KO with a Hydro Pump. They still must watch out for Freeze-Dry, however. Skarmory is immune to Earthquake and takes any other hit while also removing Stealth Rock with Defog. Ferrothorn can usually take any hit other than Superpower and retaliate with its dual STAB moves or annoy Mamoswine with Leech Seed. However, Life Orb-boosted attacks will still dent Ferrothorn. Bronzong resists Ice and is immune to Earthquake thanks to Levitate, allowing it to hard wall most Mamoswine sets. The only move it must watch out for is Knock Off, which dents Bronzong when boosted by Life Orb.
**Fake Out**: (colon) Lopunny and Mega Medicham can anti-lead Mamosine by using Fake Out to break its Focus Sash, then outspeed and knock it out the next turn, preventing it from setting up Stealth Rock.
**Status**: Mamowsine hates status, especially burns, as a burn cuts which cut Mamoswine's power in half and whittle it down turn by turn. Poison also hurts Mamoswine by shorting its already-low staying power due to Life Orb recoil.
**Fast Attackers that Tank Ice Shard**: Metagross and Keldeo, (comma) among others, (comma) can outspeed Mamoswine, tank at least one Ice Shard, and then proceed to OHKO it. Other Pokemon that will take a hefty amount from Ice Shard, such as Gardevoir, can still revenge kill Mamowsine as long as they are healthy.[/quote]