First, get the key to open the door to the fighting arena. After getting the key and opening the door, go inside and wait for the Boss to appear. Next, watch were he throws the barrels, and if they are in the correct location, shoot them back at him using the buttons on the ground.

Note: Watch out when the Boss throws a lot of barrels, you cannot shoot them back at him. Also, watch out for the coconuts that fall a little while into the fight.