There are five eggs in front of Sunnycide. Ignore Sunnycide and go for the eggs. Attack an egg until it hatches. A Yoshi will come out and start pushing a boulder on a ledge high above. Hatch five eggs while avoiding Sunnycide's attacks, and the Yoshis will launch the boulder at Sunnycide and knock him down. He is now vulnerable. Attack him with everything you have while avoiding his attacks until he stands back up. Repeat this process until he has been defeated.