There are 493 Pokémon in Pokémon Shining Diamond and Shining Pearl, but few are as difficult to deal with as Spiritomb, especially Cynthia. He has a dark and phantom typing which gives him a lot of resistances and immunities. But Spiritomb has one. weakness that you can use to defeat him.
Spiritomb only has one type it is weak to and that is Fairy. His dark and ghost typing only leaves him vulnerable to fairy attacks. If you want to exploit this, you'll need a good fairy Pokémon like Azumarill, Clefable, or Togekiss. Alternatively, you can catch a Pokémon that learns a fairy move like Moonblast, Play Rough, or Dazzling Gleam.
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Spiritomb is completely immune to Psychic, Normal, and Fighting-type attacks, so don't bother with those. It also has a good stat spread except for HP and Speed. This means you can easily outrun Spiritomb and hit him with a powerful fairy move before he has a chance to leave.
With the exception of any Fairy Pokémon or Fairy-type attacks, you can use powerful stabs to lower Spiritomb's high defense and special defense and eventually knock it down.
For more Pokemon info, check out How to Beat Fantina in Pokemon Brilliant Diamond and Shining Pearl and How to Beat Crasher Wake in Pokemon Brilliant Diamond and Shining Pearl on Pro Game Guides.
