First, go to MOSSDEPP CITY. Form there you swim to route 127 and dive everywhere until you find a place to go in the Underwater Cavern or Seafloor Cavern. In Route 127 go more to the bottom side and dive.
Second, you go through all the way and there will be some things you have to figure out. Then you'll find a place where you can't see anything because of fog. Run around then you'll find Kyogre. AQUA leader will come out and do some stuffs. Then you'll have to battle him.
If you didn't even finish finding Groudon, then here are the steps. . .
First check if you have a Magma Emblem. If you don't have, go to Mt. Pyre( Easy to look for in the Pokenav), then go to the go to the way top until you find a Grandpa and a Grandma to gether talking to someone. When there done talking go to the Grandma and she will give you the Magma Emblem. Now go to the Jagged Pass, and go down all the way and when you almost finish the Jagged Pass there will be a cave opened by the Magama Emblem go in and search for GROUDON.
After you are done with all the things mentioned above, go to SOTOPOLIS CITY, and there you'll see KYOGRE and GROUDON fight, talk to steve(Go to the place where Mart is) and he will lend you to the cave of origin, find wallce, then fly to SKY PILLAR and find RAYQUAZA. then fly back to SOTOPOLIS CITY , and seee them battle. Then talk to ARCJIE and MAXIE. then talk to Wallace now you can go to the 8th gym.
by awakening rayquaza,
to do this you need a mach bike, then you need to go to the sky pillar near pacifidilog town.
wallace will be waiting for you, he will open the door and you must complete the floors of the sky pillar, then zoom past the cracks and get to the floor that rayquaza is in, then awaken rayquaza and then it will begin to fly, then go to sootopolis city and watch the fight between kyogre and groudon.
after that talk to team aqua leader and team magma leader, then talk to steven and wallace, then the gym door will be unlocked for you to battle juan.
if you still don't understand watch this video-
http://www.YouTube.com/watch?v=Wa9WQrZ1hBs
i hope the video helps.
First you need seven badges,then go to goldenrod city and defeat team rocket tin the
radio tower.Now go to mahogany town and the man who stops you from passing wont hassle you any more. (talk to him if you want to buy a rage candy bar)When you pass him your in blackthorn city and the gym is in there.
P.Sgetting the badge is a hole other story;)
Once you Defeat Team Plasma from Dragonsprial Tower, go to route 8 and travel East. You should see Tubeline Bridge. Take Tubeline Bridge, to Route 9. Take Route 9 to Opelucid City. Listen to the boring speech and Iris will take you to her house. They will talk about The Legendary Pokemon. Then you can challenge the gym, It is in the far left corner of Opelucid City.
That's easy. In the start of pokemon platinum you'll get a town map and go to your bag, then go to key items. Use your town map to get to towns and in towns there are gyms to battle but i warn you, their tough
You have to beat the 1st 7 gyms and then go to Viridian City and the door to the gym will be unlocked.
8th gym leader
the 8th gym is closed because you have to beat all other 7 gyms and then it will be open p.s. the 8th gym leader is giovonni
Giovanni
The Viridian Gym (which is the 8th). beside that, there is the Pewter Gym, found above Viridian, after the forest.
It is the 8th gym....The last gym so just ignore until you beat the first 7
the 8th gym is ground type
Wallace is the 8th gym leader PEOPLE!!!!!!!!!!!
Volkner is the Gym Leader of the 8th gym.
8th gym leader
you just need to explore the upper floors of sunyshore city and voiala! you have the 8th gym
In the 8th gym. he is the gym leader
On every Pokemon game the 8th gym is the hardest gym.
You battle the 8th gym leader, Clair, at Blackthorn City.
To get in the 8th gym you have to go find the gym leader in the light house.
you can get to the 8th gym after you defeat team galactic and defeated 7th gym.
you beat the gym leader
go and beat the 8th gym