Instructions
Travel to Route 204 south after your victory over the first gym leader in Oreburgh City. Head to the ravaged path and use the "Rock Smash" you learned from the previous gym leader to clear the path and exit the cave.
Travel along Route 204 north until you reach the town of Floaroma. Heal your Pokemon and buy new items if needed before leaving town and traveling along route 205.
Travel to Eterna Forest and continue through it to reach Route 205. Follow the route north until you reach Eterna City. This is the location of the Eterna Gym and home of Gardenia.
Walk south through town until you reach the gym. Gardenia is standing in front of the gym and after talking to her, she will lead you inside.
Defeat the trainer in front of the flower clock to make the hands of the clock rotate to the right. Walk across and defeat the trainer on the right to make the hands rotate left. You will be able to leave the gym at this point if you first want to go heal your team or stock up on items.
Defeat the trainer on the left for the clock hands to rotate a final time and allow access to Gardenia. Use fire and flying attacks to take down her Pokemon and win the battle.