How to beat the Electric-type Gym in Pokémon Scarlet and Violet?

Iono is the Gym Leader for the City of Levincia; she is an in-universe popular streamer with two unique Magnemites on her head and an oversized jacket; taking into account the levels of her Pokémon, she is the third recommended Gym Leader to challenge.

How do you get to the Electric-type Gym?

You can get to Levincia from Artazon, but the road that leads there is blocked by Team Star's Fire Crew; the simple solution would be beating their base or a well-placed jump across the river, any of Koraidon/Miraidon's unlockable abilities help a lot to do that.

Jump to the right and luckily you land on the other side of the river

Doing the Gym Test

This Gym's Test involves you participating in one of Iono's Streams, where you have to spot Director Clavell in different places, first on the right side of the screen under an umbrella, next inside the Pokémon Center, where the Nurse should be, and third in a boat under the bridge; you'll battle one of Iono's followers after rounds 1 and 2.

Battling the Gym Leader

First Battle

Iono specializes in Electric-type Pokémon, usually weak only to Ground-type moves, though half of her team will be immune to these; electric-type Pokémon are resistant to other electric-type moves, as well as Flying and Steel-type ones.

Iono's team has Wattrel (Lv. 23), Bellibolt (Lv. 23), Luxio (Lv. 23), and a Mismagius (Lv. 24) that she will terastalize into Electric-type.

Both Wattrel and Mismagius are immune to Ground Type Moves unless you use Smack Down first; NaCli learns this move at level 10 and can be found in the South Province (Area 3); Rolycoly also learns this move and appears north of Levincia; Larvitar also learns this move at level 24, but it only appears in Pokémon Scarlet also at the north of Levincia.

Wattrel, being Electric/Flying-type, adds weakness to Ice and Rock-type moves to compensate for the Ground-type immunity; it will receive neutral damage from Electric-type moves and adds resistance to Grass, Fighting, and Bug-type moves.

Mismagius knows the Move Confuse Ray, so you might want to give your Pokémon a status healing berry just in case, especially if you plan to use Smack Down and then switch to another Pokémon.

After the Battle you’ll receive the TM048 (Volt Switch)

Rematch

On the Rematch during the post-game, Iono now has Kilowattrel (Lv. 65), Bellibolt (Lv. 65), Electrode (Lv. 65), Luxray (Lv. 65), and Mismagius (Lv. 66).

Killowattrel is the evolution of Wattrel, but in terms of resistance, everything remains the same.

Electrode does know the move Magnet Rise, which makes it immune to Ground Type moves, so Smack Down might still be advisable.

Kilowattrel and Mismagius's case remains mostly the same as the first battle, but Mismagius no longer has the move Confuse Ray, so there's no risk of the confusion status effect.