Instructions
Go to the Ice Path area of Blackthorn City in the northeast of Johto and walk around until you encounter a wild Swinub. Use Fire, Water, Fighting, Grass or Steel-type moves to wear down its health points until they show as a red bar then catch it in a Pokeball.
Assign the Swinub to your traveling party of Pokemon and use it in battles so it gains experience points. Collect enough experience points to progress the Swinub to 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.
Go into the Battle Frontier and take part in battles until you have collected 40 Battle Points. When fighting in the Battle Frontier all Pokemon are fixed at Level 50 so you will need to employ superior strategy rather than overpower your opponent with high-level Pokemon.
Go to the house in the entrance to the Battle Frontier, the Frontier Front, and head inside.
Speak to the man at the bottom right of the room and he will offer to teach a Pokemon a move. Choose Piloswine, then select the move Ancientpower. If Piloswine already knows four moves, you will have to select one to forget.
Use Piloswine in battles until it has gained enough experience points to level up again. As soon as it levels up after learning Ancientpower, it will automatically evolve into Mamoswine.