I didn't manage to get to any bosses. leftshift+s didn't work for whatever reason or I missed something. I am not a thinker so I couldn't figure out how to pass the second level :(
Sorry about that, making tutorials is hard! That segment of the game is meant to teach the player how to perform the "ground pound" attack, which is done by pressing S and Left-Shift while in air. We hope you'll give the game another try!
