After Steven gives you the Devon Scope, allowing you to make invisible pokémon become visible, you can enter the Fortree Gym.
Prepare
You can catch Magnemite and Voltorb in New Mauville. They are very helpful in this gym. You can reach New Mauville if you go back to Mauville City and talk to Wattson, who is standing outside near the Pokémon Center. He will give you the key to New Mauville. You have to go south to Route 110 and use Surf in the northern part of the route to find the entrance, which is in the northeast part of the route.
However! One of the Fortree Gym Leader's pokémon knows a Ground-type attack, which is super-effective against Electric-type pokémon. Therefore, you should bring a different type of pokémon in addition to your Electric types. Flying-type pokémon are immune to Ground-type attacks, so you could use a Flying pokémon in that case. Skarmory is an especially good choice.
Battle Trainers
In Ruby and Sapphire, after entering the gym, go to the right. Walk through the gate. You will pass by Bird Keeper Jared, who has a level 30 Doduo.
After defeating Jared, walk to the right to pass through the gate. Nearby, there is Picnicker Kylee, who has a level 30 Swablu.
After defeating Kylee, walk downward against the gate to make it rotate. Then you can walk to the right and go upward.
Up above, there are a couple of gates that you have to walk through multiple times before you can reach the next trainer. First, go up the left path and push the gate upward. Then go up, left and down so that you can push the same gate to the right. After that, walk to the gate on the right, and push it to the right. Next, go down and left, then up. Push upward on the gate, then go up, left, and down, then push the same gate to the right. After that, go down to push down on the same gate. Finally, go down, then right, then up and push left on the gate. Then you can walk all the way to the left to go past both gates.
Go up and you encounter Camper Terrell, who has a level 29 Taillow and level 29 Swellow.
After you defeat Terrell, go up, then all the way to the right, pushing the lower gate to the right. Next, go up, and push the upper-right gate to the right. Then go right, up, left, and down to push the same gate down. Next, go to the right, then up to battle Bird Keeper Will, who has a level 28 Wingull, level 28 Swellow, and level 28 Pelipper.
After defeating Will, go left until you are above the upper-left gate. Go down to push that gate down, then continue going down as far as you can go. Then go to the right, then up, pushing the lower gate up. Continue going up, then go to the right to push the upper-right gate to the right. After that, walk to the right, then go up. You can now go all the way up, then all the way left to reach Gym Leader Winona. Skip the next section to read how to defeat Winona.
Gym Leader Winona
Winona has a level 31 Swellow, a level 30 Pelipper, a level 32 Skarmory, and a level 33 Altaria. Electric-type pokémon work well against the first three, but the Altaria knows a Ground-type attack, which Electric pokémon are weak to. Try using a Flying-type pokémon against Altaria, since a Flying-type pokémon will be immune to Altaria's Ground-type attack. Skarmory is a good choice, because it is also resistant to Altaria's other attacks. Unfortunately, Kecleon is not a good choice against Altaria, because it will become a Dragon-type pokémon whenever Altaria hits it with a Dragon-type move, and Dragon-type pokémon are weak to Dragon-type moves.
After Winona
After you defeat Winona, she gives you the Feather Badge, which lets you use Fly outside of battle. You can use Fly to go to towns and cities that you have been to before. Winona also gives you TM40 Aerial Ace, a Flying-type attack.
Now you can go back to Route 120 where you got the Devon Scope from Steven.