Recommended Team: Magnemite or Poison-Type Pokemon with strong non-Poison-Type moves
Once you've healed your Pokemon and bought any items that you think you may need (have a number of Antidotes ready, it's a Poison-type gym after all), it's time to tackle the gym itself. Head towards the centre of Virbank City to find.
Inside, head down the steps until and walk over to the door until you're stopped once again by Clyde. He'll give you yet another Fresh Water and give you some information on the gym. Once you're ready, go through the doors.
You'll find yourself in a music club! The two people at the south side of the room aren't trainers, but those that are on stage are. Move around the stools and tables and walk up the stairs onto the stage.
You have to defeat the Roughneck drummer and Guitarist before you can face the Roxie, the Gym Leader and bassist. It doesn't matter who you face first, but this guide will assume that you face the drummer first.
Gym Member Battle: Roughneck
Once he's defeated, go over to the Guitarist and speak to her to challenge her.
Gym Member Battle: Guitarist
Once the Guitarist has been defeated it's time to face Roxie herself. Go and heal and restock etc., then return and prepare to face the Gym Leader.
Gym Leader Battle - Roxie
With Magnemite in your party, this battle should be easier than the one against Cheren. Make sure that your party's average level is about 13 before challenging Roxie. It's recommended that your Magnemite's a level or two higher.
Don't start with Magnemite, but instead use it against Whirlipede when you face it. Use all your other Pokemon first to deal Koffing damage until it's defeated. Roxie will have a Super Potion that she'll use on Koffing when it's HP is low, so be prepared for that.
When Koffing's finally defeated, send out your Magnemite as Roxie sends out Whirlipede. As Whirlipede only has Venoshock (which can't damage Magnemite at all) and Pursuit (which is weak against Magnemite) as its offensive moves, it can't do much damage at all.
Now simply use SonicBoom or any other damaging move until Whirlipede faints and you've won! Easy, right?
As a reward for winning, Roxie will give you the Toxic Badge! Congratulations! Furthermore, she'll also reward you with a TM, TM09. This teaches Venoshock . Once you're done talking to her, leave the gym and heal your Pokemon at the Pokemon Center.
The man that was blocking the gate to the north-north-east of the Pokemon Center is now gone and you can enter! When you're ready, do so.