Fight your way to the Top Dog Boss and kill him.
TIP: Most of the time there is fire around the place. Let him charge you. With your back to the fire and DASH (left or right) and punch him in the fire.
REMEMBER: Close to he's death there is a X button showing. This is the only way to kill him. If you didnt see it on time. It will take some more punches to show it up again. So do it as fast you can.