Things aren't going well in Saffron City. Team Rocket is everywhere, and you're not able to enter Gym Leader Sabrina's Saffron City Gym until the bad guys are taken care of. To get into the gym, first you must enter the Silph Company building and fight Giovanni and Team Rocket. Once that's done, head over to the Saffron City Gym to solve the gym's puzzle and fight Gym Leader Sabrina.
Sabrina is a
Psychic-type trainer, so
Ghost-,
Bug- and
Dark-type Pokémon or moves are helpful for taking her on. Her Pokémon are levels 37 to 43, so your Pokémon should be around the same levels. You earn the Marsh Badge,
TM04 (Calm Mind), and 4,300 Pokédollars for beating Gym Leader Sabrina.
Make sure you have powerful enough
Ghost-,
Dark-, and
Bug-type Pokémon in your lineup, or Pokémon that have moves that match these types. If you skipped catching a
Ghost-type Pokémon in the Pokémon Tower in Lavender Town, head back there to catch a
Gastly or
Haunter. (Your highest encounter rate for
Haunter is on floor 7 there.) Level it up significantly before heading back to Sabrina.
Another good option is catching one of the two sleeping
Snorlax in the game and teaching it
TM30 (Shadow Ball) They're located on routes 12 and 16. Once you've caught one of them, you can purchase
TM30 (Shadow Ball) from the Celadon City Game Corner for 4,500 coins.
Snorlax is great because it can essentially work as a tank against Sabrina's powerful Pokémon, then fight her off with Shadow Ball. This is the best option.
Sabrina also uses a
Venomoth, a
Bug-type with
Psychic-type moves, so a
Fire-type Pokémon is useful, too.
Charizard, if you started with
Charmander, is perfect. Lastly, you can also use
Psychic-type Pokémon to match hers.
Hypno, for instance, can learn
TM30 (Shadow Ball), too.
Sabrina's gym is full of warp pads that teleport you around the gym. Here's the path to take.
You face a few trainers on that pathway before taking on Sabrina.
Slowpoke (Level 33)
Slowpoke (Level 33)
Slowbro (Level 33)
You are awarded 600 Pokédollars for winning.
Mr. Mime (Level 34)
Kadabra (Level 34)
You are awarded 680 Pokédollars for winning.
Kadabra (Level 31)
Slowpoke (Level 31)
Mr. Mime (Level 31)
Kadabra (Level 31)
You are awarded 620 Pokédollars for winning.
Slowbro (Level 38)
You are awarded 760 Pokédollars for winning.
Kadabra (Level 38)
Mr. Mime (Level 37)
Venomoth (Level 38)
Alakazam (Level 43)
You are awarded 4,300 Pokédollars,
TM04 (Calm Mind), and the Marsh Badge for winning. The badge lets you command Pokémon up to Level 70.