You can't....
Sorry you can only migrate it from firered i think in emerald or i think there is a day where there is a swarm of pineco.
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it is not in Pokemon heart gold or Pokemon soulsliver. the only whey to get it is to evolve it from pinnco.
To get Forretress you need to evolve a Pineco. To get a Pineco you need a Pokemon that knows Headbutt. Use Headbutt on a tree and a Pokemon will fall out. This can be a Pineco. Ilex Forest is an example where you can come upon Pineco by headbutting treees.
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Pineco evolves into Forretress at Level 31.
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