Rock type Pokemon are strong against flying, ice, bug and fire type Pokemon.
Water type
water
rock ground flying grass
You can beat Skyla the gym leader with a higher level pokemon or with electric or rock type pokemon. And I do not reccomend using grass or bug types.
It all depeds on the Pokemon. If you has a Water Type and he has a Fire Type, you can beat him. If you have Pikachu and he has a Rock Type, then switch it because Rock Type can resist Lighting Type moves. (I have this and I can't beat Brock!)
Heres one for ya ghost types
Ground types and psychic types beat poison types.
Grass, Flying and Rock types.
rock ground flying grass
Rock beat Ice types also Steel, Fire and Fighting types.
You will need to battle all four Elite 4 trainers and beat them however how to beat them by using their weaknesses will depend on the game you're playing. Pokemon Red/Blue/Yellow and FireRed/LeafGreen the first member uses Ice and Water Pokemon so opting to use an Electric-type Pokemon is a good option, against Bruno he has Rock-type Pokemon and Fighting-type Pokemon so for his Rock-types you can go with Water or Grass-type attacks and for Fighting-types you can go with Flying, Ice and Psychic-types, Agatha you can go with Dark and Ghost-type Pokemon as well as for Psychic-type Pokemon such all of her Pokemon are either pure Poison-type or part Poison-types and for Lance his Pokemon are mostly Dragon-types so Dragon-types and Ice-types would be able to beat him and Electric-types can beat Gyarados and Aerodactyl.
You can beat Skyla the gym leader with a higher level pokemon or with electric or rock type pokemon. And I do not reccomend using grass or bug types.
Ground
with water type and rock type pokemon.
i recommend a ground or a rock type, considering they dont get hurt by electric types. Dont even try putting flying types out there though.
Rock, electric and ice.
fighting, fire, and rock.
The best way to beat her ice type Pokemon is to use fire type Pokemon. Also Ice type Pokemon don't like steel, rock and fighting types.
Rock, Ground, and Water-type Pokemon are weak against Grass-type moves. This means Grass-type moves can deal twice the damage to the listed types.