5. Boss Rush
As you'd expect, Boss Rush requires you to play through all of the game's bosses in one go and it can be found in the Extras menu after completing the game. You start with four additional lives, the upper and lower harpoon, the saw, torpedoes, and three jet thruster charges. You do not have the triple harpoon, though, which makes bosses harder than they would be if you had it.
You begin in a hub area with base stats and can choose from three upgrade crates: Repair Speed, Hull (health), and Harpoon Charge Speed. A teleport device on the far right side of the area will take you to another area to battle the current boss.
After defeating a boss, you are returned to the hub area and can choose another upgrade. In my successful run, I upgraded Repair Speed three times and Hull each other time, ignoring the Harpoon charge speed upgrade entirely. You'll also find a crate with an extra life after the Chain Gang boss.
I'll provide a video of my successful run and break down each of the bosses with a brief written guide, but ultimately, completing Boss Rush comes down to practice and persistence.
Boss 1: The Worm (0:15 in the video)
This guy should be trivial. Watch for him to telegraph where he's coming from. Get out of the way of his vertical charges and if he comes out halfway to spit out worms, get close to him and against the wall/ceiling and blast away.
Boss 2: The Fathers (1:16 in the video)
Another fairly easy boss with not much to say in terms of advice. Avoid the seahorse projectiles while you move back and forth and shoot them.
Boss 3: The Forgotten One (2:48 in the video)
You can be killed quickly here if you're not careful. Make it a priority to avoid the poison clouds as they will deal serious damage if you touch them. Once you understand his two charging attacks (one is a two or three charge burst, the other is a vertical charge after doing a flip/spin), you can get good damage in after avoiding him. Remember that there is no rush so take your time and don't over commit.
Boss 4: The Tranquil (4:45 in the video)
Start by getting behind the upper tentacles, then harpooning/sawing them. Shoot his eye, harpoon/saw tentacle three, then harpoon his heart to get to phase 2, which is much more dangerous. I like to blast his new tentacles from safety on the left (avoid falling rocks, though) to weaken them all over, then carefully go in and cut them with the saw. He flails a lot more during this phase so be extra careful so you don't get smashed. Once you cut off the top two, shoot the eye, cut off tentacle three, then finish him off by harpooning his heart.
Boss 5: The False Light (6:59 in the video)
This should be easy. Dash to the left or right as soon as he appears, harpoon him until he disappears, and repeat. Just watch out for falling boulders and bubble streams that can shove you to the side.
Boss 6: The Chain Gang (8:42 in the video)
This became infinitely easier once I realized that the saw kills the small sharks. Start by getting to the right of the main shark and try to bait him into his explosive mines (if he chomps both, he'll have about 20% health left), then start circling the area and handling small sharks with your saw. I found it best to keep moving and switch to the Harpoon once most of the small sharks are killed so you can shoot them. After clearing them out, kill the large shark by shooting him in the mouth while avoiding his charges (bait him into the mines first if he hasn't chomped them at this point).
Boss 7: The Parasite (11:07 in the video)
This boss will follow you around, then split into a bunch of tiny pieces that attempt to crash into you and knock you into hazards around the outer part of the area. You can get killed very quickly if you aren't careful in staying away as best you can from the small pieces. I found it best to do a tight circle around the large phase and shoot it continually until it splits, then dash away from the small pieces until they recollect, firing at them when it's safe to do so.
Boss 8: The Heart of the City (12:31 in the video)
Things get hectic with this boss and it feels like a bullet hell situation at times with projectiles coming from all directions. You need to carefully destroy one and then two plants to open the way to the bottom (each takes 8 harpoon shots), then shoot the boss located at the bottom when it opens up to expose a pink, interior section. Swim to the top of the area and use the saw to cut through the weak part of an upward moving crossbeam, then swim back to the bottom and again shoot the boss when it exposes the pink section again. Swim upward and use the saw to cut through the weak section of seven downward moving crossbeams, after which the boss will grab you and pull you to the area below for the second phase.
Like the previous phase, you must destroy plants to unblock the path to the bottom, after which you can shoot the boss heart repeatedly until it is killed (all while avoiding an endless barrage of projectiles).
Boss 9: The Experiment (16:01 in the video)
There may be a better/safer/more efficient strategy for this boss than I used, but I took it very slowly and chipped away at his health to ensure that I never got into a position where I would take too much damage and die. In order to damage him, you need to shoot the panel covering his heart so that it disappears briefly, then shoot his heart (the panel will reform after a few seconds). The boss constantly shoots out bloody projectiles, both randomly and after you shoot its heart, so try to avoid them as best you can. You want to always keep the boss above you a) so you have access to his weak point and b) so it won't hit you with it's occasional horizontal burst attack that deals heavy damage. The area is large and it will move back and forth from the left to the right side, so be prepared to open the door in the middle so you don't have to go over the central rocky structure to change sides.
Boss 10: The Guardian (22:54 and 26:37 in the video)
This boss has two phases and you get returned to the hub for an upgrade after defeating the first.
Phase 1: The boss will swim around the area and launch a ton of red projectiles (and the occasional small sentry that also shoots projectiles). You need to shoot its head with torpedoes to damage it. Try to keep your distance as you constantly shoot torpedoes in its general direction. It will sometimes lock on and come straight towards you, in which case a single torpedo hit will knock it away.
Phase 2: The boss will move you through four quadrants where you must avoid projectiles/hazards, starting with the right quadrant and moving you through the upper, left, lower, and right quadrant again before then moving you randomly through the quadrants until either you or it are killed. After avoiding a charged laser beam in the left and right quadrants, you get a change to shoot it with torpedoes to deal damage before it moves you to the next quadrant. The laser will track you for about 6 seconds while it charges, then it stops and unleashes a deadly blast that you need to get away from. You'll need damage the boss 5 or 6 times to kill it (once per laser cycle), with the goal of avoiding projectiles/hazards and staying alive until you get enough opportunities to shoot it.
It has four tentacles that can create problems for you, but if you shoot them with torpedoes, you'll create a weak spot that you can hit with the saw to cut the tentacle off permanently. Try to do this in your first trip around the quadrants so the tentacles don't bother you for subsequent quadrant visits (things start to get more hectic then).
The lower quadrant has poison pipes that you need to avoid. The left quadrant has a constant barrage of falling rocks. The right quadrant spawns a steady stream of small sentries. And after you get to the random quadrant part, each quadrant will spawn small sentries for the duration of the quadrant cycle, so be sure to harpoon them quickly while avoiding projectiles/hazards.
After defeating the Guardian, you'll get a results screen and an achievement will unlock:
Unlocked by 9 tracked gamers (2% - TA Ratio = 6.48)
This should be your final achievement so congratulations on another completion.
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