You can actually catch a Gengar as a Wanderer at the Giant's Cap, or from Max Raids in Giant's Mirror, South Lake Miloch, and Stony Wilderness. Try to use a Gengar if you can find one, as Haunter is inferior in pretty much every way. It obviously works in-game, as most things do, but why settle for less when you can have more?
Haunter / Gengar @ Spell Tag
Ability: Levitate / Cursed Body
- Shadow Ball
- Sludge Wave
- Dazzling Gleam
- Hypnosis / Giga Drain