Click to start the boss, WASD or Arrow keys to move, E to attack.
For some reason my project "Boss Arena 2: Hard Mode" freezes during the third boss battle, and as a result is unplayable (it might just be for me, but I tried it on multiple computers and got the same result). So I decided to make the Secret Boss (which you normally would have battled after beating the third boss in under five minutes) into a separate project. I really like this boss because it works differently than the others. You'll see what I mean when you play it. Music from Kirby Super Star Ultra, sound effects from Half-Life.