I got tagged but I didn't get notified by the tag, but here I am anyway.
The average Iwagakure Shinobi can stay awake for 10 days straight,
Reapers seem to be active all the time regardless of day and night, but it's hard to compare that with "staying awake for 10 days straight" since that's not really combat appliccable.
Plus, the thing doesn't seem to have any form of regeneration so even if it doesn't need eyes, or uh... anything, to live, it still needs blood. It will bleed out.
Admittedly, the one that kept fighting Margerit had one eye removed out of its socket and another damn near carved out. As noted by Marg: wounded, but still very deadly. Marg was using (according to Bart Torgal who saw the beginning of that fight) a jagged piece of scrap metal, and initially Marg had managed to pierce the Reaper's neck, but it managed to shake her off and keep on fighting (leading to the eye stabbing).
It took about four minutes for her to kill him (all the oxygen she had left), and she implies she did so by carving a hole through its ribcage while it was blinded and spiraling deeper into the water, in a rare showing of intelligence where the Reaper was attempting to drown its prey.
It's a fish. It doesn't have limbs
I mean, it's an alien sea creature, not a regular fish, so it does have limbs... Those spider legs it calls a face
(which are pretty strong since they can tear through reinforced titanium without much of an issue)
Reaper
scales around 320 MJ while Gabi scales about
50 MJ (in a combined effort with another character) or
77 MJ, so the Reaper is over 4 times stronger and tougher at least, or over 6 times at most (if not more).
It's also far superior LS-wise considering Reapers can easily shove around 54 m-long late 22nd century subs made mostly out of titanium (way out of date similarly sized submarines weigh many hundreds of tonnes), while Gabi is a 3-tonner (not much stronger than the Reaper's usual brand of prey, funnily enough).
Gabi is clearly smarter by virtue of Reapers being little more than mindless brutes but this is a pretty relentless killing machine that attacks just about anything in sight and can only be warded off with some food, a tactic that only temporarily works and that I don't think Gabi would use. Smaller size should help him the same way it did Margerit... but Margerit had equipment that allowed her to fight well underwater, which Gabi lacks.
I don't think pressure points would work given its alien physiology, not just alien in the extraterrestrial sense but the fact that this is an animal unlike any Gabi has encountered (unless he's encountered some other oarfish-serpent-spider-shark thingy in his story). Same would go for all those martial arts skills made to combat humans and not sea monsters from outer space.
I reckon Gabi can dodge around for a bit after surviving the initial 10 m assault (a 10 meter distance between a Reaper and a man-sized prey item is extremely small considering the Reaper is 55 m long and its face claws can extend far more than 10 meters, even when they aren't extended they're about 14-ish meters, so Gabi is pretty much right in front of the Reaper's face as if he's basically been dropped into a tank just to feed the giant predator), and may get a few good hits in thanks to his smaller size, but it won't result in lasting enough damage (considering how Reapers seem to ignore most kinds of external damage) and he's eventually going to be shaken off if he holds onto any of its fins and I don't think Gabi can endlessly hold his breath to survive a fight reaching the deeper parts of the Dunes (which are 400 meters down).
Gabi's eventually going to have to come up for air and potentially get impaled by the Reaper's spider-like mandibles or rammed and hit by the thing's horn portrusion after taking far stronger impacts than anything Gabi's endured. His techniques don't really work against something like a Reaper (I'd reckon especially since they're fighting underwater), while the beast is very much in its element and able to use it to its advantage.
