Abyss Anglers: Pressurized Eyeglass

you can sit here shooting the big guy with your harpoon for twenty minutes, then some random paladin can come up and get the last shot in and you dont get the achievement.

what worked best for me was to get directly above it, and stay far enough away that i couldn't see the counter, and shoot down. this tends to confuse it so it doesn't charge as often and keeps the sharks from tracking you.

if it charges up and you see the counter, just rise with it and keep shooting, then follow it back down.

You don't have to kill it in a single dive, I went back in immediately and it was still "injured" as I'd left it