The trick i found is very easy and based on the fact that the game pauses when you alt+tab. What you should do: every time the boss attacks and a pack of arrows appears on the screen quickly printscreen and instantly alt+tab to close the game. Watch your screenshot anywhere you like and figure a right move to counter the boss upcoming attack. Then alt+tab again and quickly hit the proper arrow key. You can hit it multiple times to be sure. Then printscreen - alt+tab again and so on.

This strategy works fine on every stage of a boss fight and very easy to execute. For stage 3 you should combo it with the above mentioned pattern guide/

You can consider this way somewhat scumbag, but it helps a lot, especially in mirror mode, as every time you die to boss you have to replay the whole level. Wich is annoying.

P.S. Sorry if such strategy is already described somewhere, didnt find such guide on steam.

Good luck!