This boss took me days of trial and error to figure out. Hopefully it will help you be better than me.

What you may first notice about this fight is that when you die, you go back a room, where you must wait to get past some electricity. It sucks, and there is no way around it. Sorry.

I would recommend that you defeat all other bosses before this one (even The Parasite). You will need every upgrade you can have. It’s a tough fight.

To initiate the fight, you must go to the door to the far left in the room. At this point, The Guardian will charge at you from the right side of the screen. This is an ideal time to shoot it with as many torpedoes as possible, as it is not attacking you fully just yet.

The Guardian can only be defeated with torpedoes, which can only be fired left and right. While this
may seem irritating, it is actually sort of helpful for getting into the mindset of this part of the fight.

You see, you only ever want to be parallel with The Guardian, so that when it charges at you with all of its tentacles out, you can shoot it in the head and it will leave for a few seconds. When above or below The Guardian, it will trap you in a tentacle cage of death, killing you almost instantly.
You can hide behind the giant blocks on the outer rim of the room, but they only block The Guardians shots. It can still just swim through them and kill you.
Simply hit The Guardian with your torpedoes enough and he will die. If you take anything away from this portion of the guide, let it be these two facts:

1. The Guardians electricity balls do less damage than the turrets torpedoes.
2. The electric zappers DO NOT hurt The Guardian. They do kill you instantly.

Once this phase is over, you get a save, which is nice.

This room is evil. You need your dodging skills en pointe, and your patience to rival Ghandi’s. You are about to die a lot.

You cannot hit The Guardian most of the fight. It takes 4 hits to kill, and those hits are few and far between. Spend your energy on avoiding the constant barrage of bullets flying at you.

This phase of the boss fight works in quarters: The Guardian uses its tentacles to separate the room up, and will move them (and you, or else you die) to different portions of the room, blocking off all but the small space you are permitted.

The order of these rotations is as follows: to the top of the screen, to the right of the screen, to the bottom of the screen, to the left of the screen, and then randomly until the fight is over (you must stay well positioned else you get annihilated by its moving tentacles.

In each quarter of the fight, there are various environmental hazards: the top has electric walls, the right has falling chompy fish, the bottom has corrosive acid spouts, and the right spawns extremely malicious drones. All sides have a saw blade going back and forth along the edges.
When on the left or right side of The Guardian, it will began by shooting bullets at you. It will soon stop, and begin charging a laser.
This laser does not hurt you yet. Just avoid it, and once The Guardian decides to fire, the boss must go into a brief cooldown. This is when you should shoot it with a torpedo.
Once you rotate to the right side of the room, The Guardian will begin to spawn drones from its mouth. These can be easily defeated with one shot from a harpoon.
With all this in mind, the rest is skill. Dodging the bullets is a matter of practice, and figuring out how to focus on the drones and the bullets and the environment is extremely difficult.

Spare Notes:
• About halfway through the fight, its partitioning arms become electric. They will kill you.
• This boss fight seems to have no music, so feel free to blast your most energetic music and zerg it.

That’s all the help I can give. If you noticed any inaccuracies in this guide, please leave a comment, and I will do my best to fix it. This game is great, but I wish that I had some help with this boss. Rate and Like if helpful please. Good Luck beating The Guardian!