Maneater

Story-related; cannot be missed.
 
The Atomic Leviathan is the final boss of the DLC and is significantly more difficult than anything you've encountered in the base game or DLC. If you go in with your normal strategies, it is likely to destroy you over and over again. It has an armor bar that you need to first deplete before you can start removing its health, and after a few seconds its armor fully regenerates, making it a potentially long fight. On top of that, once you get it down to around a third of its health left, it summons all four mini-bosses from the DLC (Bone Sperm Whale, Electric Great White, etc.) to also attack you, making things extremely hectic and deadly. There is also very little food for you to swim around and eat, plus the boss can very easily catch up to you to grab and thrash you.
 
Here is the build I used:
 

• Atomic Body - for its active ability, more on that below
• Bone everything else - for the damage resistance
• Hearty (for increased health)
• Reinforced Cartilage (for damage resistance)
• Healing Factor (to regen health)
• Brutal Muscles (to improve swim speed)

The general strategy I used is this: go into the fight with the active ability of your Atomic Body already charged. Activate it, and shoot lasers at the Leviathan over and over until it runs out. You should be able to take off its armor and a good chunk of its health in the first use. Now swim away and look for creatures to eat to refill your active ability. You will need to dash the entire time and sometimes dodge too, because the Leviathan is fast and will catch up to you and eat/thrash you. This is where the damage resistance from Bone armor and Cartilage come in very handy. Once you eat enough to refill your ability, activate it, and now since the Leviathan will likely be on you, I would shoot a laser, dodge, shoot another laser, dodge, etc., until it ran out. The second usage should definitely trigger the addition of the four mini-bosses, and the Leviathan's armor will refill. Now you need to avoid all five enemies while eating stuff to refill your ability. The Brutal Muscles should keep you fast enough to do so, just dodge like crazy too. Once you've filled up your ability again, the toughest part here is making sure all laser shots hit the Leviathan and not one of the other mini-bosses in your way. Repeat the same process as the second usage, alternating between laser and dodge, but making sure to dodge such that you move away from any enemies potentially blocking your lasers from hitting the Leviathan.
 
If all goes well, that third usage should be able to finish it off. Personally, I had an enemy get in the way of two shots and I wasn't able to kill the Leviathan with it. I charged in and bit my way through its armor and last two bites of health, but that is extremely risky to do. All it takes is for multiple enemies to stun lock you by eating you and thrashing you over and over again, ruining your attempt entirely. Play it safe and keep your distance as much as possible, especially once the mini-bosses join the fight, as things can get very frustrating very quickly. Once you destroy the Leviathan, this achievement will unlock immediately and all other mini-bosses will leave you alone and then disappear.
 
Lastly is a quick description of how the laser works, because I struggled a lot to understand exactly how to use it properly since the game doesn't really explain it. You're going to activate your ability with first, then you need to make sure you're facing the Leviathan (or any enemy you want to shoot). Hold for a second or two to charge up the laser, then release to shoot the blast. You should be able to get off a solid six laser shots before your ability drains all the way. It really becomes a matter of continually holding and releasing over and over to shoot blast after blast. I just wanted to offer this explanation since the game didn't make it clear at all how to use this laser.