Same here. I can't figure out what I should click nor how to orient the black line. It needs either a better tutorial or any visual feedback on how the supposed wave projectile that needs to hit the heart works. I did the same thing on an enemy that seemed to be the same (heart down), did the same thing (oriented line down, left click mouse) and once I hit it another time I lost.
Nice looking background though, grats on that and I hope you make a better tutorial when possible to have a probably cool game in your portfolio ; )