if you want to battle jasmine in soul silver go to the top floor of the light house and you see Jasmine with a Pokemon when you talk to her she will say something about a secret potion's surf to Cian wood when you get down at CIAN WOOD city go left and you see a sign go into the house and talk to the guy in the house then he will give you the secret potion. After you get the secret potion and defeat the Cian wood gym leader Chuck. When you defeat him talk to his wife outside the door of the gym and she will give you the Tm fly .Make one of your Pokemon learn the move fly use fly to go to Olivine city. When you reach Olivine city go to the light house the top floor and talk to Jasmine again. Jasmine will give the potion to the Pokemon and the Pokemon become healthy and then Jasmine talk to you and goes back to her gym and then you can battle her.
Olivine city
Surf from Olivine
Olivine city
In Pokémon SoulSilver, after reaching Olivine City, you have to head to the Lighthouse and make your way to the top. Jasmine will tell you how an Ampharos is ill and she asks you to go get some special medicine for it. Head to Cianwood City, get the SecretPotion from the Pharmacy there and you can battle the Gym Leader in Cianwood City first or you can return to Olivine City to give the SecretPotion to Jasmine and Ampharos and battle Jasmine and then return to Cianwood City to battle Chuck there. Go to olivine city. Head to the lighthouse. Top floor. Talk to Jasmine. Go to Cian wood . Get the secret potion. Battle the 5th gym there
you have to surf from olivine city
Olivine City, far right
you go to olivine city
surf from olivine to cianwood
It is on Cianwood City Island. You have to surf there from Olivine City. (NOTE: the gym in Olivine is actually the 6th Gym battle so go to Cianwood Gym first) Before you go to Cianwood City make sure you visit the Olivine lighthouse top floor to talk to the Olivine Gym Leader about a sick Pokemon first.
yes
you need to beat jasmine in olivine city
After beating Elite 4, take boat from Olivine City.