Fisrt, there is no reason to run two Ice moves in one set, unless one is Ice Shard. As I said, when Mamoswine run mixed sets I generally prefer Life Orbs. When using a Scarf, Mons tend to loose too much power due the split.
Fortunately (for you) Mamoswine still can pull of a 2HKO agains Ferrothorn with Superpower. Though you really shouldn't do it, here is an expample.
Mamoswine @ Choice Scarf
Trait: Snow Cloak
EVs: 116 Atk / 140 SAtk / 252 Spd
Naughty Nature (+Atk, -SDef)
- Blizzard
- Earthquake
- Superpower
- Stone Edge
Note, that while this one beats Skarmory without a Life Orb, you loose two crucial OHKOs: Virizion can only be OHKOed by Icicle Crash while for max HP Jirachi Mamoswine needs more EVs in Atk. Both are faster than Mamoswine normally; figures, who are more common targets for a Choice Scarfer. Virizion and Jirachi, or Skarmory?