Here's something straight out of the movie Thunderdome. Around the perimeter, three of four snipers with slingshots will rain pellets at your head. Ignore Johnny and take out the snipers, first. While throwing firecrackers or firing your slingshot at them, constantly move left or right, to dodge their attacks and to dodge Johnny, too.
Taking a beating and losing life? Around the perimeter, in the wooden crates, several Beam Colas are available for your consumption. Drink them when your health is low.
Once the snipers are dealt with, it's time to take care of Johnny. Your best tactic is to circle-strafe around Johnny, while firing your slingshot Or, while facing Johnny, walk to the left, shoot some shots, then walk to the right and shoot again. Repeat the pattern. Don't forget to charge your shots for a power blast.