If you chose Mudkip as your starter, evolving it into Marshtomp at Level 16 can help a lot for this gym. Avoid bringing Water and Flying-type Pokemon, as they are very weak to Electric moves. The exception to this are Pokémon that are Ground-type as well, such as a Flying/Ground
Once you defeat Wattson, you’ll receive the Dynamo badge. This will allow you to use Rock Smash outside of battle.
You’ll need to have the Pokémon that knows Rock Smash in your active party.