At the end of Stage 3 you will have to fight the boss named Mariska. The fight will consist of three small parts until you defeat her. In the first part, Mariska will be throwing balloons toward you. As long as you strafe to the right or left they will not be able to hit you. The second part is where she will be split into four and there will be a tractor riding around the arena. Don't worry about the tractor running you over, just remain strafing to the right or left with in order to dodge the balloons. The third part will have Mariska split into several bodies and there will be a hand chasing you around the arena. Just strafe to the right while shooting at her in order to take her down.