Abyss Anglers: Reservation for One

after some tries and fails I found proper solution. You need to find "dino". Rare icon on map. It swim around abyss with 2 sharks who attacks you when you hurt dino.
1) better do it with somebody. 2ppl is fine. 1 is a bit too long
2) remove all "armor" from it
3) dino disappears but you still see it in frame. Shoot exactly ONCE after that. Take ur 2500 points and press button 3 (exit).

if you shoot more than most likely you will take 2 or 3 myth items from dino and most likely you DONT get achieve. At least I failed this way 2 times with 5000 and 7500.

Getting the 1000 points from leaving early does not prevent the achievement.

However, getting multiple catch of mythic fish invalidate it.

The best way to get it is to be ready to leave early as soon as the first red message of mythic catch appears on screen.

Seems to be bugged or perhaps you can only have one catch of mythic fish. I ended up with 3 x 2500 points from killing champion pahk and nothing else, including waiting for breath to run out.

Edit: Trying a second time with just getting 1x2500 for the pahk kill worked for me. I even had to use an oxygen resupply and still got the achievement.