The last fight is unfair, not difficult, unfair. You cannot win with skill - you HAVE to get lucky. Sure, in a game like this luck is important. But in the last fight luck plain decides the game. No amount of good plays wins you the game if he just keeps getting lucky combos and you can't find any.
Once you've completed your first soft reset, there's a toggle in the Settings menu, or you can press "G" on the keyboard.