The Last Master has three forms. The first two are easy -- shoot him and kill him. The last form is difficult. You have to shoot the tank under him and destroy it. He will fall down. Make sure you hit his face when he falls down. His armor is weak when you shoot him. However, it is difficult to shoot him when he falls down because the gears are moving. Be careful, as they may crush you. When the boss gets up, the tank that is under him regenerates and you have to destroy it again. Repeat this process until he is defeated. Note: Shooting his face has no effect.