Bigger Boo

Bigger Boo is the mid-boss of World 2 in Super Mario World 2: Yoshi's Island. He is encountered by Yellow Yoshi at the end of World 2-4, Bigger Boo's Fort, starting out as a normal Big Boo. However, Kamek arrives and uses his magic to increase the size of the Boo. He does not attack Yoshi, instead floating around the room and chasing him when he is not looking, and he can still be stopped by facing his direction, though he also turns intangible, meaning that he cannot be attacked. The player must toss eggs in a way that they ricochet off the walls and into him while facing away from him, though with each hit Bigger Boo increases in size. After hitting him five times, Bigger Boo's size increases to a point where he explodes, defeating him.

Super Mario World 2: Yoshi's Island

 * Official Nintendo Player's Guide: Now here's a real back-biter. Bigger Boo closes his eyes and almost fades into the stonework when Yoshi looks at him. Turn around, though, and he'll swoop down for a dino dinner! Say "Boo!" to Bigger Boo by making the fanciest collection of bank shots this side of the world billiards championship.
 * Super Power Club Cards bio:
 * Description: This hungry ghoul will keep getting bigger and bigger until he squishes you in a corner.
 * Favorite TV Show: 
 * How to Beat Him: Look away from his big mug and bounce your eggs off the wall. Five or six hits should burst Bigger Boo's bubble. If you need more eggs, eat the bats flying around the room.
 * Shogakukan guide: まかい合うと透明になってすり抜けられる. 弱点はやっぱり背中. (When engaged, they become invisible and can slip through. The weak point is still the back.)

Yoshi's Island: Super Mario Advance 3
(The boss of World 2-4. When you face it, it becomes invisible and you can slip through. Turn your back to him and hit him with your egg five times as he approaches.)
 * Shogakukan guide: ワールド2-4のボス. 向かい合うと透明になり、すりぬけられる. 相手に背中を向け、近づいてくるときにタマゴを5回ぶつ けよう.