hints for beating the mouse see at 3 minutes on the linked youtube, but basically alternate between top right and bottom left corners luring it to each side...
The boss probably needs a bit of a health reduction...the curse of playing your game too much and getting so used to the boss fight. it was suggested by the comoposer (so that's on me)
Also If you installed the exe version you can always make it easier goto settings.ini in assets folder set allowbosskill=1 and then when you see the mouse (or the later boss) press O button to take them down to just needing 1 hit left