Back 4 Blood

This isn't a "glitchy" achievement as a lot of people have said, it's just not stated that you need to be alive and not downed when the level finishes in order to unlock this. You'll also want to be in a game with 3 other players preferably, even if they're randoms.

With that being said, this level is fairly straightforward: shoot the weak spots. The first part will have you fighting tentacles that come out of the ground. Move as one unit to take them out one at a time by shooting the inside of their mouths when they open up. Snipers work well for this part because they don't really move when they're open. After you defeat one it will go back to ground level and start spewing acid randomly, so keep clear. Once they're all defeated, jump down one of the holes all together.

Next is the Abomination's face. There will be invincible tentacles that lunge at the wall behind you, followed by the boss opening it's mouth. Simply shoot the the weak spots inside it's mouth. Assault rifles are better for this part because there will be lots of zombies and Mutations spawning in waves in this cramped room. Pay attention to the tentacles when it's health is lower, they will start lunging more rapidly.

After the boss turns around and starts moving, keep clear of every part of him, as being underneath it even slightly will insta-kill you. Split up in 2's to take both the left and right paths because you'll want to be shooting as many weak spots on it as possible. Shooting the same weak spot too much will make it burst and you'll have to find another one. You can ignore the spot under it's belly until you're farther in because you won't be able to get a shot on it.

This is very important: MAKE SURE YOU ARE ALIVE AND NOT DOWNED AT THE END OF THE FIGHT. This is not a hard fight once you know what to do, just be diligent with killing Mutations. Also, it appears the achievement won't pop until you're back at Fort Hope, and even then it took ~30 seconds until mine popped.