After talking with some of you guys, I decided to make a Frequently Asked Questions.
Be aware that it contains spoilers, so it would be better if you beat the game or try to before reading this.

1- This last boss is too hard, how do I beat it ?
Find some safe zone on the left or right side of the screen for most of his attacks, avoid staying in the center for too long. Use your lasers a lot. Do some long back and forth movements when he shoots seeking bullets at you.
I made a video of me beating him, I hope it can help :

3- I can't seam to 100% the game, what am I missing ?
Talk to Miller a lot until he repeats his dialogues, he has a lot to say. Unlock all the medals, unlock the extra scene.

4- Will there be a sequel ?
Yes, I plan on making a direct sequel. I'll have to make a much more solid rhythm system since this one is not the best. I'll also make a different gameplay I think.

5- Using a gamepad on the Android version cause some problems in the menus.
That's because the touchscreen handling system of Game Maker is not the best. It registers your touch like a computer mouse. Just touch your screen on the bottom right (to put the "mouse" out of the game) and navigate with your gamepad, this should be fine.

If you have any other questions, don't hesitate to ask.