Ground types are best against electric types.
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the best Pokemon to use against electric Pokemon is ground/rock Pokemon.
the best pokemon to use is mudkip.
Water Pokemon. You can also use rock Pokemon such as geodude or onix. Do not use grass or steel Pokemon these types are week against fire.
Its good to use whatever Pokemon you bond with best. But I suggest you use a Rock type Pokemon, or a fire type. You can catch a Darkaruma (spelling?) in the Desert Resort, which works well against Burgh's Leavanny.
Koga uses primarily Poison-type Pokemon. Psychic Pokemon would be the best bet. Ground-type Pokemon would also work, but will not be as effective against his Venomoth or Crobat.