This is a pretty grindy achievement, so I would recommend waiting until you finish the game (or the other achievements if you’re going for 100%) to try this. You also have to reach Varuna Point first to be able to attempt it in the first place.
These fish spawn in “abundance” at Varuna Point and appear only in numbers of 3 or 4 at max from what I’ve seen.
If you already completed the game you might’ve encountered them while exploring Varuna Point.
Easy low-effort solution
The easiest way to farm them is to save the game right after arriving to Varuna Point, go down in the first underwater building, kill them all, then reload the game and do it all again.
Some general tips:
- Kill the shark first, then save the game, then attempt the achievement, might save you some trouble.
- Grab an oxygen tank before starting the grind, saves some time going up on air pockets.
You could swim down first, hit the fishes until they are one hit from death (3 shots before the fatal one if you use the bow with metal arrows), save the game and when you reload, they will be one hit from death and you are spared fighting them from full HP each time. Although, you might find doing this whole process more difficult.