From Bulbapedia, the community-driven Pokémon encyclopedia.
Gengar Strikes Back is a type deck found in the Gym Challenge Strategy Guide. The deck is classified in the guide as an easy-to-build deck as it contains few rare cards, with the aim that a player of any age or skill level could complete it.
The aim of this deck is to evolve Sabrina's Gastly into Sabrina's Gengar as fast as possible. Try to leave Sabrina's Gastly and Sabrina's Haunter on the Bench until you can evolve them into Sabrina's Gengar and attach enough Energy to attack. Use your other Pokémon to stall your opponent while you get Sabrina's Gengar ready. If you can get in a few good coin flips with Sabrina's Kadabra, you might even take out some big Pokémon. (Remember, you can use Sabrina's ESP to get another chance at a coin toss.)
When Sabrina's Gengar is ready, make it your Active Pokémon and start hitting the Defending Pokémon with the Shadow Bind attack. Shadow Bind does a solid 40 damage — and the Defending Pokémon can't retreat! Use Potion and Super Potion to heal Sabrina's Gengar whenever it gets damaged, and you can Shadow Bind your way to victory!