Spiritomb and its Keystone Seal Poké-Body form a perfect partner for Stormfront Gengar, slowing your opponent down and forcing them to hold on to Trainer cards that boost Gengar’s Poltergeist attack. A single Arceus Gengar is an equally good partner, allowing you to attack and then send Spiritomb active to re-activate Keystone Seal during your opponent’s turn.
Tip: Use Blissey’s Nurse Call to keep Stormfront Gengar healthy. This will prevent your opponent from finding crafty ways to play around Gengar’s Fainting Spell, like knocking it out with Uxie’s Psychic Restore or Crobat G’s Flash Bite.