Thank you for your feedback!
The Boss has some known issues in some systems. It has to do with how Godot processes transparencies in the version we used. Hopefully we can make workarounds or it will be fixed in a newer version of Godot.
Yes the final boss may be a tad too harsh, specially if you were having trouble with the movement.
Will pass and discuss the feedback with the rest of the team. Not sure how much we will work on this game, but it will definitely be part of the things we test for and make us try better next time.