Opening the eight and final gym in Sootopolis requires you to first complete most of the game. You should at least be up to the point where you've explored Route 126 and witnessed the battle between Kyogre and Groudon. You should have also met with Wallace in the Cave of Origin and have made it to Pacifidlog City.
- Visit the Sky Pillar
The Sky Pillar is near Pacifidlog City. You'll need the Surf ability to reach it. Wallace should appear near the entrance, followed by the sounds of a powerful Pokémon awakening. You'll then need to work your way to the top of the Sky Pillar, battling some fairly strong guardian Pokémon along the way. The Mach Bike is also necessary to cross the fourth floor.
- Awaken Rayquaza
Rayquaza is the Pokémon you heard upon entering the Sky Pillar. He lives at the top. Fortunately, you won't have to fight him. Simply reaching the top causes him to awaken and fly away.
- Return to Sootopolis City
Rayquaza will fly to where Groudon and Kyogre are fighting and knock them both out of the way. After this occurs, Rayquaza will fly off. Head to the gym on the island in the center of the city and speak to Archie and Maxie. Then speak to Wallace. After giving you the Waterfall technique, he will move out of the way and allow you to enter the final gym.