You give him a super potion from the pharmacy in cianwood city
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if you mean the medicine for 'amphy' in the light house, go to the medicine man in cianwood city he will give you a 'secret potion'
You do not get to remove Amphy from the Lighthouse. Training a Mareep to get an Ampharos should be simple enough.
You catch a Marrep then evolve it at level 15 to Flaafy then at level 30 it will evolve into Amphy
Cynthia will give it to you.
go to the left of moo moo milk shop, then give the secret potion to the psyduck :)