What is spiritomb's weakness in Pokemon Diamond?

Spiritomb's only weakness in Diamond/Pearl/Platinum is Fighting -- but the only way a Fighting move can hit is if your opponent used a move like Foresight/Odor Sleuth/etc. to enable them to hit Spiritomb with that kind of attack.

Otherwise, Spiritomb has NO weaknesses.

Spiritomb is a ghost type, which is weak against ghost and dark type attacks.