After visiting the scary house and reaching Laverre City, you enter the treehouse to take on the Laverre Gym.
Fairy types are weak to Poison and Steel moves.
Battle to the Leader
In the entrance to the gym, you learn that you have to step on warp panels to move through the gym. For the quickest way to the gym leader, you should always use the warp panel that is at the farthest right of the room. However, if you want to battle all of the trainers in the gym for more experience, then in the entrance, use the panel on the left.
If you used the panel on the left, you arrive in a room with Furisode Girl Kali. She has a level 38 Dedenne and a level 38 Azumarill.
If you battled Furisode Girl Kali and want to battle another optional trainer, use the leftmost warp panel. You encounter Furisode Girl Linnea. She has a level 40 Aromatisse.
Path to Valerie
Now use the rightmost warp panels. If there are multiple warp panels on the right side of the room, use the one on the bottom right.
Eventually you reach a dining room where you encounter Furisode Girl Blossom. She has a level 37 Klefki, a level 37 Kirlia, and a level 37 Granbull.
Use the warp panel on the right, and you will find Furisode Girl Katherine, who has a level 40 Slurpuff.
After the battle with Furisode Girl Katherine, use the warp spot on the left. You'll be in Valerie's room. Talk to Gym Leader Valerie and the battle will begin.