Wario assigned him to guard one of the six golden coins in the Turtle Zone. Inside the giant whale, Mario fought the octopus and defeated him, getting the gold coin from the bag in the middle of the room.
The octopus moves slowly around the screen while sending mini octopus after Mario. To defeat him, Mario must jump on him three times. He must jump up out of the water so that when he comes back down he will damage the octopus when he lands on him.