Here we are at Gym number 2! As you come in, you’ll be stopped by the same guy at the entrance of the Pewter City Gym. This time, he won’t let you in unless you show him a Pokémon that’s at least Level 15. Surely, you must have one of those? If not, you’ll probably want to return to Route 24 and 25 and catch some more wild Pokémon for experience.
From the entrance, step forward onto the glossy platforms. Towards the left is a female swimmer who will swim down to you if you enter her line of vision. There’s no way to get around her without a fight, so get your Water-busting Pokémon ready!
We’re going to assume you’ve brought a Grass or Electric-type. Or if your partner is Eevee, it has Buzzy Buzz in its moveset. If so, go nuts. Ironically, this Goldeen doesn’t have Water-type moves, but it does have the Flying-type Peck. It shouldn’t do a lot of damage regardless.
From here, you have the choice of avoiding the next two trainers. If avoiding’s your game, instead of going straight up along the middle path, you’ll want to go around and up the side paths in a Z or 2 shape. Basically, you want the swimmers along the side to be outside of your view (and thus their view).
Another Water-type, so nothing too complicated. This one does have Ice Shard, which is an Ice-type move that always goes first (unless your Pokémon is faster and uses a similar move). But even if you’re leading with a Grass-type, so long as its healthy, it should survive Shellder.
The final Gym trainer has a Seel that knows Aqua Jet. Like Ice Shard, this is a move that always goes first (with the same caveat). But this time, since it’s a Water-type move, your opponent will gain STAB when using it. However, it’s only 50% more damage, ao shouldn’t be problematic.