Thank you for your feedback !
I added an explanation image in the description of the game regarding the "hold to charge" mechanic, since your comment and the previous two made clear that in was quite unintuitive when playing the game.
And yes the second boss can be pretty difficult to hit, especially when its low on health, but I made sure to give each bosses an attack that immobilize them for a bit, so it should be possible to touch them during this time.