When Yoshi and Baby Mario enter the area where Hookbill is found, there is fog until they reach the top where Hookbill and Kamek are located at where the fog clears. Kamek then turns Hookbill giant to fight Baby Mario and Yoshi.
To beat Hookbill, Yoshi must first throw eggs at the Koopa until he falls. Then, Yoshi must stomp on Hookbill's stomach. Keep doing that and Yoshi will beat him. Yoshi can also pound on his shell to get more eggs to use against him. After finished off, Hookbill's large shell breaks and he goes into the air and falls off the platform.