After defeating him, head north, toward the gym leader, you will be approached by another trainer. She is a Jr. Trainer♀, who uses a single Goldeen at level 19. It knows the Flying-type move Peck, along with Tail Whip and Supersonic.
After defeating her, talk to Misty.
Staryu is an easy defeat at the hands of Bulbasaur, Pikachu or Rattata. Misty's strategy is to use an X Defend to boost Starmie's Defense, while it destroys you with BubbleBeam, which has a chance to confuse even if it doesn't knock out your Pokémon. Hit it with a Leer to cancel this out, then destroy it with Vine Whip, ThunderShock, Hyper Fang or Mega Punch.
After defeating her, talk to Misty.
Leader Misty | Cerulean City Gym | 2 Pokémon | P2079 | ||||
Staryu | Water | L18 | |||||
Tackle | Normal | Water Gun | Water | ||||
Starmie | Water/Psychic | L21 | |||||
Tackle | Normal | Water Gun | Water | Harden | Normal | BubbleBeam | Water |
Staryu is an easy defeat at the hands of Bulbasaur, Pikachu or Rattata. Misty's strategy is to use an X Defend to boost Starmie's Defense, while it destroys you with BubbleBeam, which has a chance to confuse even if it doesn't knock out your Pokémon. Hit it with a Leer to cancel this out, then destroy it with Vine Whip, ThunderShock, Hyper Fang or Mega Punch.
Upon defeat, Misty will award you with the CascadeBadge. She will also give you TM11 which contains BubbleBeam. There is only one in the game, so use it only on your very best Water-type.
When you're done, exit the gym and head north onto Route 24. If you already traversed the northern routes, then go through the house in the northeast part of town toward Route 5.
Advance to Route 24
Advance to Route 5