I found this boss really frustrating and quite unlike the rest of the game. I ended up having to take notes on the pattern before I could beat the final form, and afterwards I thought I would share them in the form of a guide. While writing the first draft I noticed an even easier way than what I had used and proceed to test it in game. As such the following should work so long as the boss always follows the same pattern. A word of warning, if you want to figure it out on your own and only want hints you shouldn't read this.

This part is fairly straight forward. Press whatever arrow key moves you to the attacking arrow before it strikes. Note that the arrow that will attack you will always be face toward the center (at you).

This is where the boss can start to be tricky. Your arrow will now rotate clockwise before it attacks. It will alternate between rotations of 90º and 180º, so you will need to be one roation counterclockwise from your target or directly across from it.

It is imperative that you do this setup or the rest of the guide won't work.
You should now be at a screen where a square is at the top of the screen. Hit the down arrow key until your arrow rotates 180º to hit the square. If you do this then we can divide the key presses you need to make between 3 cases, which will simply be called case 1, case 2, and case 3.

In this case, the outer triangles (the ones attacking you) turn 90º clockwise and your triangle rotates 90º clockwise. When dealing with this case you simply move your arrow in the direction of the attacking arrow, e.g. if the attacking arrow is on the right hit the right arrow key.

In this case, the outer triangles turn 180º counter-clockwise and your triangle rotates 180º clockwise. This case is dealt with in the same manner as case 1. Simply move your arrow in the direction of the attacking arrow.

In this case, the outer triangles turn 90º counter-clockwise and your triangle rotates 90º clockwise. When dealing with this case simply move your arrow to the opposite side of the screen from the attacking arrow, e.g. if the attacking arrow is on the right press the left arrow key.

When I played through the boss followed this order for the cases 1,2,3,2,1,2,3. So basically just count up then down and then up again while apply the techniques explained above. I should also note that when I say something like "the side the attacking arrow is on", I'm refering to the location of the outer arrows before they rotate.

I hope this is helpful. If anything isn't clear or doesn't work let me know and I'll try to fix it. Good Luck!