Fantina will be in the Town with the Contests. I forgot the name of the town sorry!

Heres in depth detail about how to get Fantina into her gym. when you meet her you won't
be able to battle her so here are the steps:


Steps-

Go to Pastoria City and talk to the Team Galactic grunt next to the Great Marsh Area.
Then, go to Valor Lakefront and keep talking to him until he battles you. Walk straight
ahead until you meet Cynthia. After she gives you the Secret Potion, go to the flock of
Psyduck on route 210. Then deliver the old charm to Cynthia's grandma. Then after go back
to hearthome and you will be able to battle Gym Leader Fantina. But first you need to get
HM03. That hm is surf. First give the charm to Cynthias grandma, Then she will give you
surf. You will first need to battle Team Galactic Boss Cyrus

Fantina will be in her gym in Hearthome City. It's on the right side of town, near the
contest hall. She uses Ghost Pokemon, so make sure you bring lots of Awakenings or Chesto
Berries.