Instructions
Visit the home of your rival in Sandgem Town and speak to his/her little sister. She will inform you of Pokemon Swarms. These are daily instances during which rare Pokemon becomes available in certain locations.
Check with your rival's sister every day until she tells you about a Swinub Swarm on Route 217.
Travel to Route 217 and wander in the grass until you encounter a Swinub. Battle and capture it as you would a normal Pokemon.
Place your new Swinub into your party and raise it until it reaches level 33, at which time it will evolve into Piloswine.
Fly to Pastoria City and enter the house Northeast of the Pokemart. Speak to the man inside and he will offer to teach your Pokemon any move it may have forgotten in exchange for a Heart Scale.
Accept his offer and have Piloswine learn Ancientpower.
Train your Piloswine and the next level it gains while knowing Ancientpower will trigger the evolution into Mamoswine.