Go to Clanwood. Then Go to the building near the bottom. Inside should be a guy behind a counter. Talk to him. If you talked to the girl that was trying to heal it, talk to the guy. If not, talk to her first. He'll give you the medicine. Go back to her. This works in soul silver too.
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You can get to Jasmine but this time beat Chuck and obtain HM Fly, go back to Olivine City, Jasmine uses the secret potion to heal Ampharos and you can beat her at the gym.
Mareep evolves into Flaffy a Lv 15 then into Ampharos at Lv 30.
The best electric type Pokemon can be ether a Raikou or an Ampharos, I hope this helps.
go to that city in the bottom left of your map that has the safari zone entrance place and go in the pharmacy place and but medicine for it.
The medicine for the lighthouse Pokemon (ampharos) is in cianwood city. Get the medicine and return to olivine. Deafeating the gym will make this easier because you will be able to use fly, once you defeat the gym and walk out the lady will give you the hm for fly. Return to olivine and cure ampharos then you may battle the gym and its leader which is the person who was taking care of ampharos.