After battling your neighbor in the Tower of Mastery, Korrina tells you to challenge her in the Shalour City Gym.
This is a Fighting type gym. Fighting pokémon are weak to Flying, Psychic, and Fairy moves. They are strong against Rock, Bug, and Dark moves.
Use the Circle Pad to skate up the stairs of the gym. There is a railing at the top that you need to skate onto in order to enter the rink.
One of the trainers is Roller Skater Kate. She has a level 28 Meditite and a level 28 Mienfoo.
Another trainer in the rink is Roller Skater Dash. He has a level 30 Heracross.
There is also Roller Skater Shun. He has a level 27 Pancham, a level 27 Throh, and a level 27 Machoke.
Finally there is Roller Skater Rolanda. She has a level 27 Sawk and a level 29 Hariyama.
Skate to the Middle
Use the Circle Pad to get on the rail from the left or right. You'll end up in the middle, and Gym Leader Korrina will appear. Talk to Korrina to start the battle.