Double click the green flag. Choose which boss you'd like to face, then beat the boss before they kill you. Click or hold down the mouse to shoot and use WASD or the arrow keys to move. All bosses are possible to beat. After losing or winning to a boss, click the green flag to play again. If the defender's auto cannons don't show up the first time, then click the green flag again, select the defender again, and they will be there.
