Misty has water Pokémon on her bench. So if you picked Bulbasaur in the beginning, she's a cinch. But...if you picked Squirtle, you might want to teach Squirtle TM01(which you'll find in Mt. Moon), which contains Mega Punch. If you picked Charmander(which should have evolved into Charmeleon, like mine:), you have a problem. Since Charmander is weak to water, Charmander is weak here. But if you have a Pikachu, that's not saying much since electric is good against water. However if you don't have a Pikachu, go to Nugget Bridge and level Charmander up with the trainers there.

Or, you might want to catch an Oddish(Red) or Bellsprout(Blue) in Route #24. They're grass types and they have an advantage against water.

At Route #25, there's some trainers there, so you can level up your Charmeleon(if, you picked a Charmander) there. You can also gain TM19(Seismic Toss) there so you can teach it to your Charmeleon then use it to pound the stuffing out of Starmie(I picked Starmie because he's the one I have a problem defeating).

If you win, you get the Cascade Badge, which allows Pokémon up to L30 obey and allows you to use HM01, Cut anytime.