37 - Olivine City (Second Visit)
When you arrive from using Fly, you will be in front of the Pokemon Center. Take a moment and heal up your Pokemon as they are probably feeling that last fight. After that, head to the right to reach the Lighthouse. This tie, you can simply take the elevator up. This speeds along the process nicely. Talk to the girl, Jasmine, nearby to give Amphy the Potion. The Ampharos makes quite a scene when it feels better. As soon as this is done, you will be able to challenge her in her Gym.
There is chance about this time the Baoba will call you and let you know that the Safari Zone is open as well. As it is near Cianwood, it may be something to do in a little bit. For now though, it is time for the next Gym Challenge.
From the Lighthouse, head to the left. You will want to turn upwards between the Poke Mart and the Pokemon Center. From there, follow the path to be lead right to the gym.