Instructions
Defeat the Magikoopa who guards the prison. Dodge his minions or launch star bits at him to occupy his attention until you get close enough to the Magikoopa to use your spin attack. One hit will defeat the Magikoopa, causing the shield around Yoshi's egg to lower, freeing dinosaur once you jump on top of his egg.
Defeat the final boss, King Lakitu. Lakitu will fire Spinies at Mario from a distance that makes it impossible to hit him with traditional attacks. Lakitu will also attack Mario using his sunshine attack ray.
Attack Lakitu using Yoshi's tongue to grab the Spinies Lakitu fires at Mario before they hit the ground. Yoshi will hold the Spinies in his mouth, giving you time to aim before shooting them back at Lakitu. Yoshi can also spit the Spinies in his mouth at the ones that hatch on your platform to clear more space for Mario to dodge Lakitu's attacks.
Aim at where King Lakitu is going, not where he is at, to hit him with the Spinies. After you hit him two times, Lakitu will speed up his attacks and move quicker. Lakitu's quicker pace will make it more difficult to hit him with the Spinies. But by avoiding his attacks and leading your shot, you can hit him for a third time, defeating him and ending the level.