Much like the Fuchsia City Gym, the Saffron City Gym will be yet another difficult challenge for trainers to face. Most of the trainers found within this Pokemon Gym will own Psychic Type Pokemon, and these types of Pokemon are only weak against against Dark Type Attacks and Ghost Type Attacks. Unfortunately, there aren't that many Dark Type Pokemon and Ghost Type Pokemon found within the Kanto Region.
There are a few Ghost Types found within the Pokemon Tower back in Lavender Town; however, these Ghost Types, including Gastly and Haunter, are actually weak against Psychic Type Pokemon because they are part Poison Types as well. Because of this, your best shot at defeating the Saffron City Gym rests in having extremely high level Pokemon. Make sure to train your Pokemon as much as you can before facing this Pokemon Gym; the stronger your Pokemon, the better yours chance will be at defeating this Gym on your first go around.