Serin Fate

If you are here to check how hard/long it is/takes to unlock this achievement... it is by far the most tedious achievement I have ever unlocked. It took me 23 hours of optimized grinding to get it. Other people gave up after listening to 2 full audiobooks and 40 hours. It will be an absolutely miserable time and you shouldn't go for it. If you like to 100% games... avoid this game.

With that out of the way, there was almost no information floating around for this achievement. I did a lot of digging through old Steam discussion archives and the official Discord and can give the following advice to get this achievement:

• the only 2 catches (which is me + the only person that has a confirmed catch and commented) were obtained using the Power Bait. So only use that bait.
• I would highly recommend getting the fishing hat which reduces trash items obtained from fishing. The hat can be obtained in the top right of Nymbus Beach above the save stone
• It must be raining (NOT storming (Thunder)) to be able to fish the spined ling (confirmed by the dev)
• You have to actually get the spined ling through fishing. Obtaining it through the Spell "Sea Trap" won't count toward the achievement
• The Nymbus Isles are rumored to have a slightly higher chance of obtaining the spined ling. I recommend fishing on Nymbus Beach (starting area with the crab race)
• I tested many spots and the bottom right seems to be the most efficient spot with no rocks or plants which would obstruct the pathing or lead to dead ends
• The bottom right is where I got my spined ling and is also the spot where the only other person on the internet that commented and has a confirmed catch got his spined ling.
• I tested stacking potions of luck and dex many times (up to 6 or 7 for each simultaneously) and they had no effect of increasing the chance for rare fish
• Many people rumor that the fish that will bite for only a millisecond is the spined ling. You will encounter this about 1/3 of every bite. I tested a lot with this and got about 100 bites through anticipating the bite and got only 1 rare fish from this (a crab. No spined ling). I can safely say that every successful time you hook a fish can lead to a spined ling, as I got mine through a normal bite. Don't waste your time as I did with this.
• Equip a Sun Amulet to make the rain last longer. For me when I was beginning, the rain only lasted for 15 minutes. After a few hours, I put on a Sun Amulet and the rain lasted for 27 minutes which results in half as many resets and therefore a higher efficiency. Depending on your situation you could also equip/swap for a Moon Amulet.

So for the method itself:
Buy yourself the fishing Rod + lure from the Shop and craft yourself a Deep Lure as well as around 20 Power Bait (Farm the slimes on the Beach east of Midtown for materials if needed). Fly with your turtle to the starting island of the Nymbus isles called Nymbus Beach (it's the one you first arrive at with the whale and has the crab race). Hope that it is raining, otherwise, wait until it rains. You can cast the Spell "Home" or fly back to your base with the turtle once it reaches 6:00pm and go to sleep to skip some time. It can take you a few hours until it finally starts raining...I warned you that it will be a miserable time. In the waiting time, you should get the fishing hat in the top right of Nymbus Beach and farm or craft a Sun Amulet if you have none. Once you got it to RAIN (and only rain. Thunderstorms and normal weather result in a 0% Chance for spined ling) you save at the save stone in the top right. After this point, you NEVER EVER save again. And now the fun part begins. Go to the bottom right corner of the area and start to fish... a lot... but only until it stops raining. When it stops raining ESC out of the game to the main menu and reload your save. Rinse and repeat until you get your spined ling or die of old age.

Good luck my friend. You will need it.