This city's gym specializes on Bug-type Pokémon. Having a party with a Fire-type and Flying-type are recommended. Having a Pikachu on hand will also be helpful as some of the Pokémon in the gym are also part Flying-type.
Upon entering the Gym, jump on the pole by simply heading near it and you'll land on what looks like a web. From the center, head to the lower right to battle the first trainer.
Youngster David(Prize Money: P240)
From David, head to the lower right, lower left, left, up, up, left, left, left, left until you meet the next trainer.
Youngster Zachary(Prize Money: P240)
From Zachary, head up, up, left, up, right to reach the next trainer.
Lass Charlotte(Prize Money: P240)
Head up to battle the gym leader.
Gym Leader Viola (Prize Money: P1920)
Before challenging Viola, do not place a Fire-type as your first Pokémon as Viola's first Pokémon, Surskit is a Water/Bug type. You may either use a Flying-type such as Fletchling or Pidgey for Gust or Peck, or Pikachu for Thunder Shock. After defeating Surskit, Viola would send out Vivillon. Vivillon is a Bug/Flying-type so Fire, Flying and Electric are good against it. I would recommend sending out your effective Pokémon that has the highest HP as Vivillon's Infestation might catch you off-guard. Infestation is a move that damages your Pokémon for 4-5 turns and the affected would not be able to flee or switch out.
After winning, Viola will present you with the Bug Badge, the badge that lets Pokémon obey you in battle until LV30. She will also give you TM83 Infestation. You may head out of the gym through the staircase behind Viola.
As you exit, you will be told to head up to the Pokémon Lab in Lumiose City. To head to Lumiose City, go to the right and then north at the first corner. You will be stopped by Alexa, Viola’s sister. She will give you an Exp. Share.