This boss was easy more out of accident than anything else:
So, there was I, playing Super Paper Mario and it's time to go and crush one of the last bosses, Bonechill. So, I grab Luigi and I'm off to deafeat him. When he lowers his head for the first time, I jump on it.
The boss immediatly raises his head and, for some reason, he takes Luigi with him. Luigi gets stuck between two ice shards on the ceiling and starts boucing non-stop on the head of Bonechill, who gets killed after two seconds or so.
I still have no idea what happened, really. It was way too fast.