A video showing every single possible boss (not including midbosses) that can be defeated using Ghost Kirby.

Some notes:
Pressing B as Ghost Kirby will give instant invincibility.
Even when possessed, an enemy hit a certain way will always be stunned during knockback until it touches the ground. This means that if you get hit when possessing an enemy, and there's only a pit below you, you won't be able to jump or do anything and your host will just fall and die.