As you said it is best to use dark types since they have a resistance to Ghost. Obstagoon is obtainable early and a pretty good Pokémon. It is also a normal type so it has immunity to ghost. It’s moveset could be
I’ll leave you to figure out items and EVs. If you have moveset similar to this you should be able to defeat quite a few Pokémon.
Other Pokémon include Malamar, Morgrem (You probably can’t get Grimsnarl yet) and not recommend but useful, Theivul.
I’d say Obstagoon is best since it can be useful in later game