Instructions
Catch a Swinub in the Ice Path area in Blackthorn City in the northeast of the Johto region. Use it in battles until it gains enough experience points to reach Level 33 and it will automatically evolve into Piloswine.
Go to the Battle Frontier area in the west of Johto, to the northwest of Olivine City.
Enter the Battle Frontier and fight in battles until you have gained 40 Battle Points. All Pokemon in Battle Frontier fights are set at Level 50, so you will need to use tactics and strategy to beat your opponents rather than overpower them.
Go to the house in the Frontier Front area at the entrance to the Battle Frontier and head inside.
Speak to the man at the bottom right of the room and he will offer to teach one of your Pokemon a move. Select your Piloswine, then choose AncientPower from the list of available moves and pay him 40 Battle Points. If Piloswine already knows four moves you will have to choose one to replace.
Leave the Battle Frontier and use Piloswine in battles until it has gained enough experience points to level up. At this point it will automatically evolve into Mamoswine.