I played thru the online version and finished it.
The boss' visual feedback was very broken. It would get hit and stay flashing (though I think that was intentional) but would also sometimes take damage (or at least play a damage sfx). Because of all that it took longer than necessary to learn the boss' movements and act accordingly. Initially, I would get stuck in a corner and die instantly from contact damage.
One the plus side, I liked the use of the extended dash.
Overall, a nice game. I look forward to more.