First you have to go to Cinawood city and then go to the pharmacy. When you talk to the guy he will give the secret potion, beat the gym (if you want to), go back to olivine and fight the trainers until you get up to the light room. Give the secret potion to jasmine and she will unlock the door.
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Surf to Cianwood and visit the pharmacy there.
You can only obtain an Ampharos if you level it up from a Flaaffy. A Flaaffy can be caught on Routes 42 and 43.
You do not get to remove Amphy from the Lighthouse. Training a Mareep to get an Ampharos should be simple enough.
You need to go to the bottom floor and take the stairs and ladders. Yes it means going against a few trainers and actually working but Its for a badge. (Hope it helps)
I have HeartGold and in Cianwood theres a store there and go in and he'll give you the SuperPotion or something like that. Give it to Jasmine and then Ampharos will light up.Pokemon soul silver the Pokemon in the Olivine City Lighthouse Needs medicine, you can get the medicine in the Cianwood City and you get it in the pharmancy next to the medical center