The Cerulean City Gym specializes in water-type Pokemon. In order to enter, you must show a Pokemon at level 15 or higher. If you’re having any trouble with the gym, grab the Bulbasaur from the woman west of the Pokemon Center and start capturing Pokemon to level up quickly. Your partner Pokemon and the Bulbasaur should be enough to sweep the gym with electric- and grass-type attacks.
There are three Pokemon Trainers that will challenge you to a battle if you take the direct path. You can always avoid them by taking the side paths, but that will mean you won’t earn as much money. The three Pokemon Trainers are listed below.
Misty is the leader of the Cerulean Gym. She is stronger than all other Trainers here, but only by a little. Continue using your Partner Pokemon and Bulbasaur to make quick work of her team. They will attack with water- and psychic-type special attacks, so be prepared to strike first or withstand the attacks. Misty’s special move is called Scald. This is a strong water-type special attack that might leave your Pokemon with a burned condition, which reduces the attack stat.
Defeat Misty to receive P3,040 and TM29 Scald. You will also receive the Cascade Badge which allows Pokemon up to level 30 to obey you in battle. With that in hand, you can either go north to meet up with Bill on routes 24 & 25, or if you’ve already completed that quest you can now go south to route 5.