Kill all his pkmn
In the first part of the game you should catch a shroomish. its hard to raise so put it in the daycare. daycare only lets you grow up to 20 levels. after it grown for about 18 levels take it out and put it back in. it should evolve at i think at level 23 to 25 into a grass and fighting type Pokemon. Breloom is what its called. once your at the gym (the 8th one you're in now) take it and use grass moves. grass in this gym is better than electric because some of his Pokemon are water and electric which doesn't affect those Pokemon. but watch out for his last Pokemon. electric is not super effective but does good damage to his last Pokemon. his Pokemon use ground type, water, and electric. All of those are not very effective. But again his last Pokemon knows ice beam which is super effective against grass. use electric on his last Pokemon. you don't even have to train your Pokemon that much. only up to about level 45 or higher. My manectric is level 43 and killed most of his Pokemon. Get a plusle or minun under cycling road because they learn Thunder at level 38. (Strongest electric attack). Manectric learns it at level 61. that's alot of training. oh yeah, you can use an ground type moves against the water and electric Pokemon Whischcash that's his name. Watch out, if you use Geoude, Graveler or Golem an Sandscrew you might die from water pulse.
Conclusion:
Use grass type for the first 4 Pokemon.
Then use ground moves or grass moves on Whishcash.
You have to teach one of your Pokemon the HM Dive after beating the Mossdeep City Gym. After teaching Dive, go to the dark blue water ring around Sootopolis (where the bright white rocks are) , that's where you use dive. After useing Dive, you should see an enterance, enter it. After entering it, go to the center of the room then stop using Dive. Now you should be in Sootopolis City /END
In Pokemon Sapphire, Sootopolis City is only accessible using the HM Dive. There is a spot outside of Mossdeep City where the player can Dive and then resurface in Sootopolis City.
(improved answer) you go in front of sootoplis city follow it on you map and you see dark spots in the water use dive and you see a cave entrance go in and press b
U have to see the fight between groundon n kyogre n raquaza first.
You need to go through the hidden mine and venture till you see light. He is there.
have a Pokemon about level 54
hes in sootopolis city gym
He is the sootopolis city gym leader.
sootopolis city
Emerald: He is the champion of the Elite Four; Juan is the Sootopolis gym leader. Sapphire/Ruby: He is the gym leader of Sootopolis.
Sootopolis City. The gym leader is Juan.
You get it from Sootopolis's Gym by defeating the Gym Leader.
Wallace
He is at Sootopolis city...you can battle him only after the sootopolis incident
The 8th Gym Leader in Pokémon Sapphire can be found in Sootopolis City.
Go to Sootopolis.
He's the Gym Leader in Sootopolis City.
you go to sootopolis city