Use Fighting type attacks
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Slaking: A normal type which is heavy and sleepy. The perfect match would be Primeape. Primeape has an ability which enables it to stay awake when its foe tries to make it sleep. Since Slaking usually knows Yawn, this is a good advantage. Primeape is also a fighting type which would be super-effective against Slaking. Also, if you teach it Low Kick- which is more effective on heavier pokemon- you have a winning stratedgy against Slaking. Just make sure it has a high enough level.
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.
super effective is bug
psychic pokemons and attacks are super effective