I'm glad you enjoyed it a lot! You're probably the first to get an S rank (if no one else claimed otherwise so far).
A speedrun timer is a great idea for end-game players as a skill expression! I will look into that should I proceed with the game development further down the line.
Admittedly I kept the last attack as is because from the playtests sessions, the players actually gave positive feedback despite its difficulty. It may have been the dark souls effect with the satisfaction of defeating a difficult boss. I definitely will look into incorporating at least an easier difficulty for people to ease into the game much better. That or I could just ride the current wave, I think the retry mechanic gives plenty of handholding to players as is.
Thank you for your comment!