the eighth gym is located in sootopolis, which is southwest from Mossdeep. the enterance to Sootopolis is underwater, so you'll need a Pokemon who can use the HM DIVE to get in. the enterance is in a maze of underwater trenches, so it will take some searching to find it. (dive in the dark blue water)
The 8th gym is in Sootopolis City. When you get there ( Under water in a cave by route 127 and 126) you will tour around and find Steven. Steven and The Gym Leader will take you to a cave. You'll go in and find your way around but you should find Waterfall ( A TM) on your way into the cave. It's pretty easy to find. Once you get there you should battle the pokemon. Once you've defeated or caught him you can go into the gym and fight the people and The Gym Leader. Hope this helped!
Find the key at a under water cave
you can get it before the 8th gym
GO to the spear pillar and go inside climb to the top and your there but it has to be after you defeat the 8th gym!
beat the 8th gym
sootopilis city
Go to Sootopolis.
...... stewen in emerald isn't a elite champion. there is wallace.
sootopolis city
i go to mossdeep city
in the fortree city
somebody gives it to us after beating the 8th gym
He is at Sootopolis city...you can battle him only after the sootopolis incident