There are 493 Pokémon in Pokémon Brilliant Diamond and Shining Pearl but few are as tricky to deal with as Spiritomb, especially Cynthia’s. It has a dark and ghost-typing which gives it a lot of resistances and immunities. But Spiritomb does have one weakness that you can use to defeat it.
Spiritomb only has one type that it is weak to and that is fairy. Its dark and ghost-typing leave it only vulnerable to fairy attacks. If you want to exploit this, you will 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.
Spiritomb is completely immune to psychic, normal and fighting-type attacks so don’t bother with those. It also has a good spread of stats except for HP and speed. This means that you can easily outspeed Spiritomb and hit it with a strong fairy move before it has a chance to go.
Short of any fairy Pokémon or fairy-type attacks, you can use strong stab moves to whittle through Spiritomb’s high defense and special defense and eventually take it down.
For more information on Pokémon, check out How to beat Fantina in Pokémon Brilliant Diamond and Shining Pearl and How to beat Crasher Wake in Pokémon Brilliant Diamond and Shining Pearl on Pro Game Guides.
