By training your Pokemon.
level it up by one level after it has learned ancientpower
Get a combee from the pokewalker and level it up so it evolves.
Just teach it AncientPower, then level up
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.
level it up by one level after it has learned ancientpower
To get Pidgeot, you must level up your Pidgeotto to level 36.
The elite four
Just teach it AncientPower, then level up
You level it up until it is lv.30
Level it up until it is Lvl 30
Get a combee from the pokewalker and level it up so it evolves.
Level it up with high friendship
Level Cherubi up to level 25. It will evolve into Cherrim.
Suicune is at Level 40 in HeartGold and SoulSilver.
breed it
No, you must level up pupitar to lv51 or trade.