The Worm: The Worm is fairly simple and shouldn't give you much trouble. When he first appears, shoot him to make him back off and the fight will begin. You can see where he is because the rocks rumble, indicating his position. When he spits his babies at you, you can move to the lower corner under him and unload harpoons into him. The only thing to watch out for is when he moves between the ceiling and the floor. He will one-shot you here.

The Fathers: Keep your distance and try to stay above them. Look for openings between attacks to move. You can kill their babies, which spawn at around 50% health. The middle platform is actually part of the background and you can move through it. People have said that you can kill one of them, but I don't think it's true. It doesn't matter which one you hit, as there is only one health bar

The Forgotten One: Keep your distance and avoid the noxious clouds. His bones will replenish his health, and you can destroy them with the saw (from The Fathers). Don't spend too much time destroying the bones because he spawns them fairly often. He is extremely dangerous when he rushes you. Don't let him get too close! You can also start hitting him before the fight begins.

The Tranquil: Make sure you have the Thruster and Saw before beginning this fight. Watch her top tentacle. When it is lowered in front of her (NOT coiled back!), rush in and destroy it at the root. There are safe zones at the top of the ceiling and at the lower right just above her head. Destroy the top tentacle near the root (hit a section 4 times, then chop it off with the Saw), and then destroy all of the other tentacles in descending order. Attack her eye to open her mouth, and then attack the mouth. Phase 2 is pretty much the same, except that her tentacles move faster and there are falling rocks. I would recommend first keeping your distance and sniping the top tentacles and destroying them and then go in for the attack. Be patient here.

The False Light: I actually fought The Chain Gang before fighting The False Light, but it doesn't really matter. This is the gimme boss. For being a terrifying creature, she really isn't hard at all. All you need to do is swim left and right, be aware of the falling rocks, and thrust away when her terrifying maw appears in the background. She'll chomp at you, and give you plenty of time to inflict ~15% damage each time. Be aware that her food will hurt you when they rush away from her, and vents will spawn that will push you back into the school of fish. Avoid these, unload on her when you can, and the Lantern will be yours. I killed her on my first try.

The Chain Gang: This boss is pretty tough, especially if you don't have the Lantern. The lighthouse in the background will go dark periodically, so be alert. The feeding frenzy spawns to the left off screen. What I did was avoided the big shark and concentrated on killing the smaller sharks. The big shark will kill the smaller sharks as he passes through them. Beyond the smaller sharks, it is very possible to move right into the two mines tied to the big shark's tail, which will kill you instantly. Once you kill all of the smaller sharks, the big shark is incredibly easy. Shoot him in the mouth to kill him.

The Parasite: This boss isn't too bad. Thrust away when it gets too close, and look for openings when it splits apart and swarms you. Try not to get cornered.

The Experiment: This boss is pretty hard. It has a giant beam cannon, and a turret that shoots blood at you. The beam and the big blood balls are one-shot kills. My strategy was to only attack it when it was on the left side, and to back out when my health was too low. Stay under it, keep moving to avoid incoming blood bullets, and concentrate fire on the mechanism and the open wound.

The Heart of the City: This boss is really hard.

Phase 1: Attack the plants to remove the vines. The first plant is easy. Keep moving to avoid its fire, and the blue bubbles the big bad is shooting at you. When you destroy this vine, there is one below it with two plants. To make matters worse, there are flying Venus Fly Trap creatures that will swarm you. You need to be fast. Kill the fly traps as they approach, and stay low at the plants. This fight is basically a bullet hell game. After destroying the vines, you can damage the plant when he opens up. Avoid the spore plants that fire from the walls. After you damage the boss enough, take out the saw and thrust your way to the breakable vines when the nettles come down. You will need to repeat this final process twice, at which point the Heart of the City will grab you and take you into its maw....

Phase 2: You are now inside it. This phase is exactly like the middle of phase 1, except it's worse.because there are flying Venus Fly Traps *everywhere*, and the plants are shooting at you. Shoot the plants and the flying traps, break the vines, and when/if you reach the bottom, hit the pink bulb with harpoons to finally kill this invasive plant.

The Defiler: Don't even try to shoot it. Simply stay ahead by using the Thruster and get out of its way if the tunnels give you a chance to. In the end shoot the opening mechanism. Without the Thruster you'll have problems escaping, so make sure to get it before.

The Guardian - Phase I: It's only vulnerable to torpedoes. You have to hit the front part for damage. This fight is mostly about dodging while firing torpedoes. Don't go to the upper or lower parts of the map, because you won't be able to effectively use your torpedoes there and there's a chance you get cornered and die. Float in the middle and move to the left or right side when needed. The Guardian circles and from the middle you'll always have a chance of getting in its way and place a torpedo. If you're close enough it's even chasing you, so just fire one torpedo backwards and if you hit it, it'll turn around. In-between the Guardian will spawn drones that'll fire laser at you, so keep moving, dodging and fire some torpedoes their way until you get the next chance of hitting the boss. Be patient and you're good to go to phase II.

The Guardian - Phase II: The Guardian will force you into one section of the map and periodically turns 90 degree. Stay in the quadrant, don't touch the tentacles, walls and hazards there. Concentrate on destroying the shorter tentacles. Hit them once with a torpedo and then cut the resulting weak-spot with the saw. Once all four short tentacles are gone it'll be much easier. When you are in the right or again in the left quadrant, the Guardian will charge its laser. It doesn't hurt you while small, so draw it to the bottom and cross it upwards before it fires. After that the Guardian needs a cool down and that's the only time it'll be vulnerable to a torpedo, so hit it now.
After a while, also small drones will spawn, so shoot them with harpoons when they appear, until the laser game is back. Don't touch anything, inflict damage and eventually, you'll be the last man standing.

You can actually kill the Fathers individually, even though they share a health bar. It's a good tactic to focus on one of them so there will be less stuff to avoid after one is dead.

This guide was really helpful (much better than the one somebody wrote at Steam). Thanks.

I would add, thats its not necessary to get thruster to fight Tranquil, you can first swim in safe upper-right corner, shoot eye from here, then cut tentacles, shoot mouth and repeat. Its would be kinda long, but possible.

Also, shield you get from giant squid will help a lot in fight with Forgotten. Its smart idea to try to collect as much as possible upgrade crates before dealing with Forgotten, since his cave gets littered with hazardous crap very quickly.

Thruster is absolutely must have to fight Parasite, which is way too fast for you without it. Parasite itself is optional (he doesnt drop any critical upgrades, just crates), but killing him opening early access to secret Triple Harpoon.