I went back to Shalour City after training at Reflection Cave for a while and I quickly wandered over to the Shalour City Gym after I healed my team at the local Pokemon Center. Once inside, I had to move around the skate park-esque Gym puzzle. It required me to stake around while battling against Gym trainers in order for Gym Leader Korrina's location to be accessible. I used Golduck and Aerodactyl for many of the battles against the Gym trainers's various Fighting-type Pokemon. I beat all the Gym trainers eventually and then a rail that led to Korrina's spot at the center of the Gym was finally revealed so that I could challenge her. I healed my team with items before that battle though and I also saved the game.