Electric is weak against Ground-type attacks. (Super effective to Electric-type Pokemon) Electric, Steel, and Flying-types are not effective very well against Electric.
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Ground type Pokemon can beat Electric type Pokemon easily.
Dark type can beat ghost.
Rock, electric and ice.
fighting, fire, and rock.
The Petalburge Gym uses Normal Type Pokemon so you should take pyschic Pokemon or fighting type Pokemon if you want to beat them quickly.