For Ruby and Sapphire you simply go to the Elite Four but for emerald you go to the elite four beat them then you unlock the battle frontier.
in sootopolis city
Sootopolis city, were Kyogre and Groudon are rampaging.
Go to Sootopolis City and to catch it go back to Sky Pillar top.
You go to Sootopolis City. There you will see Groudon and Kyogre fighting, and Rayquaza goes to stop them. Then he goes back to the Sky Pillar.
The eighth gym is in Sootopolis city which is near route 126 and 127 (you have to dive to get there). First you have to find Wallace in the Cave of Origins in Sootopolis city and tell him that you think Rayquaza is in the Sky Pillar. Then you have to find the Sky Pillar (route 131 through a space in the rocks) and battle Rayquaza. It will then fly away. You then go back to Sootopolis city where Rayquaza will stop Kyogre and Groudon fighting. Then you can go to the gym there(:
The Sootopolis City Gym door is unlocked after you retrieve Rayquaza from the Sky Pillar.
First find the sky pillar and get 2 the top and bring your Mach bike to get to the top. The sky pillar is after Pacifidlog town and before the enterance to Sootopolis city. When you get to the top of the sky pillar rayquaza will be at the top but it will fly away when you see it. But when the two legendary from ruby and sapphire stop fighting go back to the sky pillar and catch rayquaza.
Go to sky pillar which is located on route 131 go to the top where you see raquaza and then he will fly to sootopolis. get out of sky pillar and fly to sootopolis and then.....
Cave of orogin in sootopolis city and the sky pillar left of dewford town(surf).
He is standing inside The cave of Origin (Found in Sootopolis City)
It becomes catchable in Sky Pillar the next time you go there after completing the Sootopolis City event.
Go to Sootopolis.