You dodge. If you can't manage this then get gud.

As you walk into the boss room, a square of hard hitting blinding shots surround your character. You can attempt to escape but it'll zoom back onto you while taking a chunk of your health. The square will start to move and your main job is to stay in the centre of the box. While you're navigating to dodge the square, the boss will shoot 2 rotating paralyzing shots in your direction, and a few slow straight shots that do a fair amount of damage and are easy to dodge. For this phase I recommend staying near the bottom of the arena.

The warden will start to fire out a flurry of mirror shots that will reverse your movement. Wait until all the shots have been released, bunching them up is the easiest way, and move out of the way last second as the first half of the released shots fire toward you. If you're moving it will shoot to where you are heading. After you dodge the first half, watch out as the second half of shots will fire towards you in the same manner.

Phase 1 and 2 will repeat until about 8000ish health is left on the boss.

Once you get the boss's health low enough it will bring out the windmill of death. It will shoot out a cross of hard hitting ♥♥♥♥♥ boys. Once it extends to full length, it will start to spin counter clockwise around the boss. If you try to brute force the phase, you might be lucky enough to not die for a single round. The trick is to hide in the bottom left corner. You'll have to tank mirror shots coming from the corner but they don't do much damage. Once you survive long enough, it'll repeat to phase 1.

The boss will die pretty quickly.

if not, you suck