Level up while it knows ancient power
You must evolve a Piloswine to have a Mamoswine.
A Mamoswine evolves from a Piloswine which evolves from a Swinub.
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.
First evolve it into Piloswine then level Piloswine up with Ancientpower as one of its moves
It doesn't evolve at a certain level, instead, in order for Piloswine to evolve into Mamoswine, Piloswine has to be taught AncientPower
teach piloswine ancientpower and level it up
Piloswine evolves into Mamoswine when he learns AncientPower, sometime after Level 33.
Piloswine evolves into Mamoswine if leveled up knowing AncientPower
Piloswine does, it evolves into Mamoswine.
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.
Make piloswine learn ancientpower and then level it up at any place
In Pokémon Glazed, you can evolve Piloswine into Mamoswine by leveling it up while knowing the move Ancient Power. Once Piloswine reaches level 33 and has learned Ancient Power, it will evolve into Mamoswine. Be sure to check your Piloswine's moves to ensure it has learned the required move before leveling up.