The only way I know of to get a super effective hit on spiritomb is to have a Pokemon use foresight on it and then attack it with a fight type move.
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super effective is bug
Fire :)
Dark Pokemon, with the exception of a few, including, but not restricted to Spiritomb, Sableye, Drapion, and Honchcrow, are all weak against Bug- Type and Fighting-Type moves.
Water and Grass attacks are super effective against Onix.