Off the top of my head: Fire type moves.
There may be more, but I have forgotten them.
The other type is Electric. However, Skarmory is also doubly weak to Ground attacks if it is holding an Iron Ball or the move Gravity is used on it.
Chat with our AI personalities
Ghost & Flying. I believe that's it! Not dark or bug because although they're super-effective against Psychic, they are not-effective against Fighting. And not Psychic, because though it's super-effective against Fighting, it's not-effective against Psychic.
Water and Grass attacks are super effective against Onix.
super effective is bug
psychic pokemons and attacks are super effective
Rock, electric, ice, and psychic types are super effective against Crobat.