I made a video guide on how best to combat the final boss:
This is the text guide:
-First of all, the machine gun never needs to reload nor runs out of bullets, so just hold that trigger down.
-Aiming/zooming is a bit of a double-edged sword here, it is useful for bullet placement but is very shaky, so if you can't control the shake, give it a try without zoom, it surprisingly useful.
-First Wave (First time you kill the boss):
-The first half of this wave is relatively easy, do your best to aim at his core, as it's his weak point.
-Shoot even if you can’t hit his core, damage is damage and may buy you just enough time to get the kill, especially if you struggle at shooting.
-Once his HP is at half, you see a brief cutscene and he begins to attack you with his outer 'eyes'.
-You need to destroy all the outer eyes quickly, by shooting them of course, I don't recommend aiming here, as it can be very shaky.
-Do your best to shoot the ones glowing purple, it will cancel their attack, in saying this, it's not easy and if you are quick, it's not likely you'll die.
-Once they are destroyed, shoot the core, ending the first half.
-Second Wave (When the boss resurrects):
-This is the more demanding part, you need to be non-stop shooting his core the entire time, missing the core for more than about 15secs total will make it almost impossible to win.
-Zooming is tricky here, you'll see that I do it a few times but it is crazy shaky, if you can manage, don't zoom, you'll get more hits in
-At the time I killed him, he was no more than 3 seconds away from him insta-killing me, so that's roughly the time frame you have to end him
-The video was my second attempt, so you can gauge how much practice you'll need (not much)
-I really think it's important to decide, “do play I with or without zoom?” It will help you play more smoothly