Mamoswine doesn't evolve either way because it doesn't exist in Pokemon LeafGreen. It evolves from Piloswine who evolves from Swinub, but Piloswine can't evolve into Mamoswine in LeafGreen.
teach piloswine ancientpower and level it up
It evolves at level 33. Level it up again with the move ancient power to evolve it into a mamoswine.
lvl it up knowing ancient power
Piloswine doesn't evolve into Mamoswine by level. It will evolve into Mamoswine by leveling up while it knows the move AncientPower. Swinub can be bred to have this move, or you can go to the Battle Frontier (after the Johto Elite 4) and use the move tutor there to teach it to your Swinub or Piloswine.
You must evolve a Piloswine to have a Mamoswine.
Mamoswine doesn't evolve either way because it doesn't exist in Pokemon LeafGreen. It evolves from Piloswine who evolves from Swinub, but Piloswine can't evolve into Mamoswine in LeafGreen.
You can't catch a Mamoswine but to get one you need to get a Swinub train it to about level 33 and it will evolve into a Piloswine and get a Heart Scale and teach it Ancient Power and then level it up one level and it should evolve into Mamoswine.
To evolve Pilowswine into Mamoswine your Pilowswine will need to learn the move Ancientpower. Then, it will evolve.
teach piloswine ancientpower and level it up
To get mamoswine you need to obtain swinub and evolve it into piloswine.Then lvl up your piloswine till it knows ancient power.lvl it up a level and bam itll evolve into mamoswine.
First evolve it into Piloswine then level Piloswine up with Ancientpower as one of its moves
Piloswine does, it evolves into Mamoswine.
Evolve it from Swinub. Swinub is found in Ice Path.
It evolves at level 33. Level it up again with the move ancient power to evolve it into a mamoswine.
Level up while it knows ancient power
lvl it up knowing ancient power