Olivine city.
The Lighthouse is in Olivine City.
Take the elevator at the bottom of the Lighthouse
olivine lighthouse and route 27
The Lighthouse is not in Kanto. It is in the Johto region. It is located on the southeast corner of Olivine City, overlooking the sea.
Go to the city with the lighthouse and then walk to the left.
Get the secretpotion from a guy there and go back to Olivine in the lighthouse and cure the Ampharos in lighthouse and then battle Jasmine
there isn't a pool
Ask the pharmasist in cianwood city to give you one then give it to the lighthouse Pokemon
level up your pokemon, and try as hard as you can to avoid some people
talk to jasmine in the lighthouse at olivine city and then go to cianwood and talk to the pharmasist
You must go get some medicine for her from cainwood, and bring it to her at the top of the lighthouse. Then she will return to her gym.
You find it in the Glitter Lighthouse in Olivine, or in the Celadon Dept. Store.