With the shotgun, aim for the hitman behind the cars. Ignore the hitman behind the wall for the moment if they cannot hit you. This will result in the car exploding, killing all hitmen. The high traffic might also cause a chain reaction of exploding cars. If done correctly, this reaction could last until the other hitmen arrive and thus cause the arriving hitman car to be set on fire. When the reaction stops and other hitmen arrive, use the burned cars for cover to prevent them from all shooting you at once. However, after about ten to twenty seconds worth of explosions you might get a one star wanted level.