Similar to Brock, Misty is a significant step up compared to the last time you fought her. As before though, you should have a pretty straightforward time if you have some solid Grass and Electric-types.
First into the swimming pool is Golduck, which Misty must have evolved from her Psyduck. This one can use Scald for STAB and to potentially burn, Ice Beam to hurt Grass-types, Psychic to damage Poison-types, and Light Screen to weaken special attacks. Try and KO it fast with a strong Grass or Electric-type.
After this, Misty may send out Vaporeon. This Water-type Eevee can use Hydro Pump for STAB, Blizzard to wreck Grass-types, Shadow Ball to surprise Psychics and Ghosts, and Quick Attack to finish off weakened foes. Vaporeon is fairly bulky, so be prepared to eat an attack from it.
Once again, Misty has her trademark Starmie. It can use Hydro Pump and Psychic for STAB, Dazzling Gleam to batter Dark-types, and Thunderbolt just in case. Healthy Electric-types are your best bet, unless you have a non-Poison Grass-type in the wings.
Gyarados is Misty’s strongest Pokémon and it shouldn’t be underestimated. Her Gyarados can use Waterfall for STAB, plus Outrage, Crunch and Iron Tail to hurt a variety of targets. Electric-types are its bane, so try and save one for this very moment. Otherwise you may have a difficult battle.
Finally, we have Dewgong. This one can use Waterfall for STAB, and Iron Tail because… well, that tail has to be good for something, right? Anyway, it’s also carrying Aqua Jet and Ice Shard. Grass-types are ideal, but you’ll need to anticipate an Ice Shard coming your way. Otherwise, go with Fighting-types.