Instructions
Walk around in the Icy Path, which is west of Blackthorn City. After about 10 wild Pokemon encounters, you will come across a Swinub.
Throw an Ultra Ball at the Swinub to catch it. If you have a full party, the Swinub will go to your Pokemon Storage System. Otherwise, it will be the last Pokemon in your party.
Train Swinub against wild Grass-, Flying-, Dragon- and Ground-type Pokemon. Since Ice-type moves are super effective against these Pokemon, Swinub will gain experience faster. Stop training Swinub once it reaches Level 33, which is when it evolves into Piloswine.
Buy one Heart Scale from the Prize Counter at the Pokeathlon Dome. The Prize Counter only sells Heart Scales for 1,000 PokeDollars on Wednesday.
Speak to the Move Relearner in Blackthorn City, and give one Heart Scale to the Move Relearner.
Request that the Move Relearner make your Piloswine remember Ancient Power.
Fight wild Pokemon and Pokemon Trainers with your Piloswine. Now that your Piloswine knows Ancient Power, it will evolve into Mamoswine once it levels up.