I found this boss fight to be quite difficult. So here is some additional info to the other guides.
The boss has 3 phases.
Phase 1: only shooting of black balls.
Phase 2: shooting of black balls and occassionally hitting with his octopus arms
Phase 3: fast hitting with his octopus arms, occassionally shooting of black balls
Phase 2 starts after first third of energy is gone. Phase 3 starts after second third of energy is gone.
For phase 1 & 2 throwing 2 debris into the eye should do it. You can also catch the black balls an throw them back! But they have to be quite close.
Phase 3 is a real bitch. Mainly because there will be so much holes in the floor and you have to look upwards all the time. Most of my deaths came from falling.
You will also have to use dash (
) more often here to evade the octopus arms. This will drain your energy and you will have to wait to throw again. But I found throwing still more successful than shooting.
You don't have to aim perfectly to hit the eye. If you don't hit the white you will still decrease some of his energy.
Try to change between a fast throw, dashing and sprinting around (click in
). During sprinting look at the floor and let the energy fill up.