Gengar is my favorite card in the Pokémon TCG. This is one iteration of a Gengar deck.
Start with Battle VIP Pass. Lead with Gastly if possible. Spam Arven and Skyla to get trainers to evolve the Gengar and Dusknoir.
Going more in-depth, Spell Tag is used as insurance for your dusknoir to ensure a return KO with Creep Show on the following turn. I don't need Radiant Alakazam because of Sinister Hand, especially in combination with Dark Invitation, which is guaranteed by using Surprise Box. Dimension Valley is key to getting away with less energy, and in a pinch you can attack with Dusknoir (Burning Shadows) to take a KO if needed. An idea setup is getting a Gengar active, a Gastly, Haunter, or Gengar benched ready to evolve, and 1 or 2 Dusknoir (Burning Shadows) and a Dusknoir (Boundaries Crossed). If there's anything I'm missing, please leave a comment!
I'm having trouble with deck space here, so if you have any suggestions, please recommend cards to take out too, because deck space is tight. I have no room for a Radiant Pokemon or an ACE SPEC card, or even more VS Seeker. I'd also like to add another Dimension Valley, but 3 should be fine.