You can't catch Mamoswine in that form in SoulSilver, but you can catch a Swinub on the Ice Path in SoulSilver, and evolve it to Piloswine at level 33, and then level it up after it knows the move Ancient Power.
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The code for Mamoswine is POKEMONDOTCOM.
Swinub evolves into Piloswine at level 33. After that Piloswine can evolve into Mamoswine if it levels up while knowing the move AncientPower.
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 get mamoswine in Pokemon leaf green because it doesn't exist in leaf green
If you want Mamoswine from FRLG? Catch the Swinub on FRLG first. Them Transfer to DPPt or HGSS from Pal Park.