I always thought thought a giant Klout would be a good idea for a boss since it constantly tries to bump you, here's my idea.
You start on a floating platform, there are other platforms that float upwards & downwards with some red Buzzes that navigate to each of the platforms you are on, if you jump on the platform that is floating down precisely then the red Buzzes stay stationary, that is until you jump on the platform floating upwards. The giant Klout will follow as well, only to bump into a wall causing the barrel to start breaking, when that happens, Klouts' speed will increase depending on wich floating platform you are on, whether the platform is floating upwards or downwards. The red Buzzes follow you again repetitively until the barrel starts to break open and Klout is defeated, keep jumping on either one of the platforms until at the right moment you throw a barrel that appears on one of the floating platforms, you throw it at the right moment when Klout's almost about to break out of his barrel, throw the barrel, he dies.
That is my idea of a boss.