Misty is a tricky opponent who shouldn’t be underestimated, even at this early stage in the game. Psyduck isn’t too bad, although it can use Confusion to hit Poison-types for super-effective damage. That’s most of the Grass-types you can use.
But similar to Shellder’s Ice Shard, Psyduck doesn’t gain STAB and if trained well, your Grass/Poison-types should be able to outlast Psyduck. If in doubt though, lead with an Electric-type–well, just Pikachu unless you’ve traded.
Finally, Misty has a Starmie, which is the real terror. This evolved Starmie gains the Psychic-type and has a powerful Water-type STAB in Scald, which has a good chance of causing Burn. It can also use Psywave to hit for 9 to 28 HP worth of damage.
Against Starmie, Grass-types are highly recommended. Poison-types don’t need to worry because, like Seismic Toss, Psywave cannot inflict super-effective damage. That said, if you used your Grass-type against Psyduck, you will need to be careful of their health.
If in doubt, your partner is a good choice, but they need to be wary of Scald. Pikachu can obviously deal some solid damage with Zippy Zap. Meanwhile even if Eevee doesn’t one-shot Starmie with Buzzy Buzz, it will inflict paralysis, making the fight easier.
Should you manage to stand your ground against Misty’s crashing waves, she’ll concede her Cascade Badge, as well as TM29 Scald. Originally, she gave Bubble Beam, but this is so much better. In addition to dealing impressive damage, it has a good chance to Burn, greatly impeding physical attackers.