The track title "Chop U!" sounds either like an Unusual Euphemism or some sort of helicopter pilot institution. The track itself is awesome.
That One Boss — Envy, one of the Rivals that appear at the end of st. 5. Suffice to say that the usual guides for a 1CC explicitly instruct you to make Zenovia Airport the fourth stage, so you can structure things to fight Blunt and Sobut, but not Envy. That's how nasty it is. It mostly has to do with Envy's lasers being nightmarishly fast no matter how low the rank is.