Only water, grass, and ice. (Hippowdon is one of the few Pokemon I can think of that are only Ground types.)
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Water and Grass attacks are super effective against Onix.
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.
Hippowdon is a Ground-type Pokémon and is weak against Water, Grass and Ice.
super effective is bug
psychic pokemons and attacks are super effective