You can't reduce a pokémon's level.
haunter has to be traded to evlove haunter evolve into gengar in level 42
i beat that gym with pokemon in their high 20s but i had a full team at that point, ideally you want your pokmon to be mid level 30s.
Level 34.
Level 38.
level 27
level 18
Level 25.
Level 35.
Level 36.
Level 36.
He will start at level 30.
Level 40