Dark, and ghost.
Ghost types are weak against Dark and Ghost type moves.
Ghost-type Pokemon are weak against Ghost and Dark types.
Ghost type is weak to Ghost type and Dark type moves.
Ghost types are weak against other Ghost type moves along with Dark type moves.
Ghost types are fundamentally weak against ghost or dark attacks.
Dark, and ghost.
the answer is the . . . . . . . . . . . . . . . . A B R
When attacking, Ghost-Type Pokémon are strong against Psychic-Types and other Ghost-Types, weak against Dark-Types and Steel-Types, and useless against Normal-Types. When being attacked, Ghost-Type Pokémon are strong against Bug-Types and Poison-Types, weak against Dark-Types and other Ghost-Types, and invincible against Normal-Types and Fighting-Types (barring the use of a move like Foresight).
The pokemon that are strong against psychic are bug, dark, and ghost.
Gardevoir is weak against the Ghost, Poison and Steel-types.
I believe Dark and Ghost types are weak against Ghost types. Ghost types may not be, but I think they are.
Ghost types are weak against Dark and Ghost type moves.
Ghost and Dark types., (Yes, Ghost is weak to itself).
Ghost-type Pokemon are weak against Ghost and Dark types.
Ghost type is weak to Ghost type and Dark type moves.
Ghost types are weak against other ghost types and dark types so use those types to defeat that elite four member.