First get the pepper spray (upgraded is best) then whole it in the middle of the map. Keep holding it there. With your other hand, while the boss is alive, press the random weapon button in the top left corner. Now left go of the pepper spray it should stay there and keep spraying the boss. You can repeat the process and get more on the screen at once. I once got 10 at the same time. Don't have any pop bottles on the screen at the same time as your pepper spray. They should stay on the screen until you exit the game.