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level it up by one level after it has learned ancientpower
Just teach it AncientPower, then level up
Get a combee from the pokewalker and level it up so it evolves.
Level Cherubi up to level 25. It will evolve into Cherrim.
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.