Hint: You need to get close to his face for him to be vulnerable.
In-depth solution: Probably not the only way to beat him, but I would attach myself to the wall directly above his head (his orbs couldn't hit me there), and cast the charged light. If there's a mini squid thing I'd rail on him quickly, then jump directly down in front of his face and get out quickly, all the while shooting. His tongue sticks out, and my bullets hit him. Takes quite a while as only a few hits in each time, but eventually it got there.
