I'm guessing 'Earth type' is 'Ground' types?.
If it is, Water types, Grass types and Ice types.
So try too get these Pokemon: Samurott, Vileplume, and Weavile.
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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.