Poison and Fighting-type Pokémon should not be used in this gym as far as possible as they are weak to Psychic-type moves. If you have caught a Gastly back in Lavender Tower, it will be very helpful if its level is high enough to outspeed most of Sabrina‘s Pokémon, as well as has already learn the move Shadow Ball. If not, get Pokémon that has learnt Dark-type moves like Bite, Crunch etc. Some examples are Rattata, Gyarados, Meowth or even Squirtle. Sabrina‘s Venomoth is the odd one out and it can be deal easily with Rock-type or Fire-type attacks.