You will face General Buttons and Fluffinstuff at the end of the third hub world Holiday Canyon.
It's not a particularly difficult boss fight - fluffy will bounce around the platform you are stood on and try to squash you. Then, fireballs will shoot across the screen that you can easily avoid by double jump and spinning. Finally, several bombs will be thrown onto the platform, and you will have to spin one of them into General Buttons to damage him. Repeat this another two times (it gets slightly harder each time) to defeat Buttons & Fluffy and win the achievement.