Dark, and ghost.
Ghost types are weak against Dark and Ghost type moves.
Ghost, Psychic, and Ground
Ghost-type Pokemon are weak against Ghost and Dark types.
Giratina is weak against: Ice: 2X Ghost: 2X Dragon: 2X Dark: 2X
Dark, and ghost.
Ghost types are fundamentally weak against ghost or dark attacks.
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Gardevoir is weak against the Ghost, Poison and Steel-types.
Ghost-type Pokemon are weak against Ghost and Dark-type moves.
Ghost types are weak against Dark and Ghost type moves.
The pokemon that are strong against psychic are bug, dark, and ghost.
Bug, Ghost, and Dark.
Ghost, Psychic, and Ground
I believe Dark and Ghost types are weak against Ghost types. Ghost types may not be, but I think they are.
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).
Ghost-type Pokemon are weak against Ghost and Dark types.