Instructions
Go to the Ice Path area between Route 44 and Blackthorn City in the east of the Johto region.
Walk around in the Ice Path dungeon until you encounter a wild Swinub. Use Fire, Steel, Water or Grass-type attacks to wear down its health points and catch it in a Pokeball.
Assign the Swinub to your traveling party of six Pokemon and use it in battles until it gains enough experience points to reach Level 33. At that point, it will automatically evolve into Piloswine.
Go to the Battle Frontier in the west of Johto and go inside. Fight in battles until you have gained 40 Battle Points.
Leave the main Battle Frontier area and go into the house in Frontier Front. Talk to the man in the bottom-right of the room; he is a Move Tutor who will teach "Ancientpower" to your Piloswine for 40 Battle Points.
Teach "Ancientpower" to your Piloswine and use it in battles until it gains enough experience to level up. As soon as Piloswine levels up after learning Ancientpower, it will automatically evolve into Mamoswine.