By the way, her name is Fantina.
Considering the fact that she has Ghost-Type Pokemon, you can take a big risk and use a Ghost-Type Pokemon because like Dragon-Type, it is so powerful only it and another Type and beat it. Anyway, you can use a Pokemon like Drifloon or Murkrow/Misdreavis (Depending on game. Murkrow - Diamond. Misdreavis - Pearl), or use Psychic Pokemon such as Meditite or Chingling.
How to get:
Drifloon - Outside the Valley Windworks all day on a Friday.
Murkrow - Diamond/Misdreavis - Pearl- At night, inside and outside the Lost Tower(Below Solaceon Town)
Meditite - Outside Mt. Corronet, East of Enterna and many other places.
Chingling - Mt. Corronet (Between Enterna and Celestic town AND Cycle Road/Orebrough and Hearthome)
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Ghost and dark type
the pokemon you could use to defeat her is electric types or rock and ice
with using a rock or ground type Pokemon, the best Pokemon you can use to defeat a electric type is rhyperior ok!?
teach a flying Pokemon defog and use it
You will need to get an Action Replay and use the Wild Pokemon modifier code.