Sunnicide is the boss of the Egg Factory in Yoob's belly. He'll start off surrounded by eggs, so start smashing them. Every time you break one, a Yoshi escapes and goes onto the top screen. When you've freed 5 Yoshis, they'll push a Chomp Rock onto Sunnicide and knock him over, revealing his weak spot inside his mouth. Use jump attacks or Cannonballers to attack it, and repeat until he's fried.
Sunnicide's attacks are quite easy to avoid. When he grabs an egg, wait until he throws it and jump over it. When he tries to suck one of the Bros into his mouth, clout him with your hammer when you're close enough. When he's fallen over, he'll pull a rope and drop one of the Bros into a hole. Time your jump just right to avoid this attack.