Dark type have weakneses to:
Fighting
Bug
You may also need to find out whether the dark type Pokemon has a dual type for example:
Spiritomb is also ghost meaning it has no weaknesses
Murkrow or Honchkrow are part flying as well meaning an electric or rock movde can take it out
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Dark and ghost type moves.
Dark types beat ghost Pokemon, as do those of a higher level with the fire type.
If you are talking about Jasmine you just need a fighting type pokemon or a pokemon that knows a fighting type move. Or a fire pokemon or a pokemon that have fire type move is fine. And if you are talking about Sabrina she is a psychic type master so you will need a bug/ghost/dark type pokemon or a pokemon that have the move type.
Riolu is a Fighting type pokemon.
I would choose a Dark or Ghost type
Dark type can beat ghost.
ghost
Psychic
You can't. There is no such thing in any legit version of Diamond or Pearl.
Fighting and bug type moves are super effective against the dark type.
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Dark and ghost type moves.
any dark type move
to beat the third gym leader use an ground and flying and water and dark type it worked for me trust it!!!!! I pitty da fool!
Get lots of fire type Pokemon! hope it helps!
Bug, Ghost or Dark.