All right! Mamoswine is one of my favourite Pokemon to use in VGC, and quite possibly the best anti-metagame Pokemon right now. It's an incredibly hard counter to Thundurus and Landorus, being able to beat the infamous Double Genie combination on its own. It also beats Mega Salamence and non-Shuca Heatran. Here's the set I use:
Destroyer of Genies
-Icicle Crash
-Earthquake
-Rock Slide
-Superpower / Knock Off
EVs: 4 HP / 252 Atk / 252 Spe
Jolly nature
Item attached: Choice Scarf
Ability: Thick Fat
Icicle Crash is obviously the move you're going to want to use most of the time: it can OHKO all Landorus-T almost all Mega Salamence even at -1, and less bulky Thundurus if you're not Intimidated. Earthquake is always a great move, especially since most teams only have the Genies to switch into it. Rock Slide is for Charizard, Volcarona, and flinches, though it is pretty weak being a 75 BP non-STAB move. The last move is mostly up to choice: Superpower does heavy damage to Kangaskhan, though it doesn't OHKO and then you're locked in at -1 Atk/Def. Knock Off is always nice, especially in a best-of-3 situation. A Jolly nature along with the Choice Scarf ensures that you outspeed Mega Salamence and Adamant Scarf Landorus in order to bop them.
Other options:
Freeze-Dry can be used to hit Water-types, but Mamoswine loses to bulky waters anyway and would rather have a partner take care of them.
Ice Shard has priority, and is pretty good on non-Scarf variants. It's pretty weak otherwise, though.
Stone Edge is stronger than Rock Slide and isn't blocked by Wide Guard, but it doesn't have the Flinch chance, requires slightly more prediction to use, and has that nasty 80% accuracy.
Fissure can be interesting in formats that allow it; while it's way too unreliable to use if you're serious about winning, it's fun to spam if you're just playing for fun. You probably won't make any friends, though.
Icicle Spear can be used over Icicle Crash if you're worried about accuracy or Focus Sash, though it's less reliable overall.
A Life Orb set with Icicle Crash, Earthquake, Rock Slide, and Protect fits very well onto Tailwind teams due to Mamoswine being a natural counter to Thundurus.
Partners:
Mamoswine is a great supporter for Charizard Y: Charizard gets rid of defensive Steel-types and weakens bulky waters for Mamoswine, and Mamoswine removes most common Rock Slide users so that Charizard can do major damage. Calm Mind Cresselia is a great third member since Mamoswine has a tendency to force a lot of switches, which gives Cresselia the chance to set up. Using a Thundurus of your own can also be beneficial because you can Taunt Sableye and co. before they can Will-o-Wisp you.
Countering Mamoswine:
Mamoswine works by putting pressure on Landorus, Thundurus, and other big-name Pokemon and forcing them to switch or pay the price, so Fake Out from Kangaskhan or Scrafty can stop it dead for long enough that a Landorus partner can get a Rock Slide off. As for beating Mamoswine itself, burns are your best friend. It can't touch Rotom-W, who forces it out with Hydro Pump or Will-o-Wisp. Sableye has Prankster Will-o-wisp which stops it cold. Heatran gets special mention since, although it loses to Mamoswine 1-on-1, it can switch into a predicted Icicle Crash and then win against Scarf variants. Steel-types that can take an Earthquake such as Scizor, Ferrothorn, and Aegislash can also win against it.
So overall, Mamoswine is fantastic at countering VGC 2015's top threats, but can't really do much outside of that, so it should be used to pave the way for another Pokemon to sweep.