First of all, Thanks for your playing!! And sorry my late Reply.
Answers the questions
1. how do you kill the last guy? With the black health bar?
=> I can't figure out your saying, maybe I assume that guy is Boss. If that guy is Boss you can kill him by Attack key as you did kill other enemys.
When I play the game after your comment, but I couldn't find 'the black health bar'. Sorry for my bad answer.
are you saying this guy?