In Virbank City, someone will mention that he found a Magnemite in the Complex that worked well against Roxie's Poison-type pokémon. You should go catch some pokémon in the complex before you challenge Roxie.
Go south from the Virbank City Pokémon Center to reach the complex. Just outside of the complex, there is an X Accuracy in the grass.
Catch Some Pokémon
Now continue south to go into the complex. You can catch Magnemite and Koffing in the grass of this area. These two types will work well against Roxie, so be sure to catch some.
You can also find Growlithe, and if you have Black 2 you can find Magby, or if you have White 2 you can find Elekid in the grass as well.
Get Those Workers Fired Up Before They Get Fired
Talk to the guy near the entrance of the complex to find out that the three new workers are not very energetic about their jobs. Agree to find the workers and give them some encouragement.
Just south of that guy, you find Youngster Masahiro, who challenges you with a level 13 Lillipup.
If you go south from there, you see some stairs leading up to a walkway. Across from those stairs, on the right, there is a narrow passage that leads to an area of dark grass. At the north of this area, a nerdy guy talks to himself about the distillation tower. He walks away when he sees that you heard him.
Go back to those stairs that lead to the walkway. The nerdy guy is nearby. Talk to him and he will say some stuff about the smokestacks. Then he walks away again.
Now go up those stairs up to the walkway area. You can walk along the red pipes, but if you stand still on the pipe or walk in the wrong direction, you will fall down to the lower level.
Go north on the walkway area to find Worker Mitchell. Talk to him to battle him. He sends out a level 12 Patrat and a level 12 Elekid. After the battle, he will be more energetic about his job.
At the northwest part of the walkway area, you find Youngster Waylon. He has a level 13 Patrat.
Jump down from a red pipe in the upper-left area to fall down to where you see another worker. This is Worker Nathan. He sends out a level 12 Riolu and level 12 Lillipup.
Now go south to the southern area of the complex. There is a grassy area here. Talk to the brown-haired girl at the bottom. She is Lass Daya. She has a level 13 Woobat.
If you talked to the nerdy guy two times earlier, he will be here. Talk to him and he will talk about the gas containers. Then he gives you an Ether because he is happy that you listened to all the stuff that he said.
Now go back up to the walkway area with the red pipes and go to the southeast. You can reach the third worker this way. He is Worker Isaac. He has a level 12 Magby and a level 12 Koffing.
Go south from Isaac to find TM46 Thief.
Now that you have defeated all three workers, go back and talk to the guy near the entrance. He gives you TM94 Rock Smash.
Before you leave the complex, be sure to catch Magnemite and Koffing. They will help out a lot in Roxie's gym.