This guide will be focused on those trying to earn this achievement through 'Quickplay' with random players online. If you're planning on completing this using the 'Solo-Offline' method, then refer to
Invincible666's guide over on 'Act 1 Cleaner'.
OVERVIEW;
Act 4 will be the easiest of the 4 acts to complete on Nightmare difficulty cause its only one level. You'll have to fight 4 tentacle mutations above ground before reaching the main boss underground and once you defeat both stages of it, you should unlock the achievement. Pretty straight forward and simple act so this can even be done without a full team of players. I was actually able to complete this with just one other player its that simple.
DECK BUILD;
Ridden Slayer (+20% weakspot damage)
Reckless Strategy (+30% weakspot damage, -5% damage resistance)
Hyper Focused (+50% weakspot damage, - 40% move speed while shooting or meleeing)
Ammo Mule (+75% ammo capacity, -5% move speed)
Large Caliber Rounds (+7.5% bullet damage, +200% bullet penetration)
Down in Front (while crouching, you dont take or deal friendly fire damage, +10 health)
Tactical Vest (+30% rifle ammo capacity, +10% damage with assault rifles and lmgs)
Two is One and One is None (can carry a primary weapon in secondary slot, -25% swap speed)
Canned Goods (+45 health, -25% stamina)
Hunker Down (while crouching gain +10% damage resistance and +40% accuracy)
Front Sight Focus (+20% accuracy, +10% weakspot damage, +15% aim speed, +15% recoil control)
Steady Aim (every 0.75 seconds you ads, gives +10% recoil control [up to 3 stacks], +80% aim speed, -50% ads move speed)
Surplus Pouches (-5% health, +1 team offensive inventory)
Box O' Bags (-10% health, +1 team support inventory)
Hydration Pack (+35 Health, -15% ammo capacity)
DECK NOTES;
This deck isn't that great outside of using it for the sole purpose of this Act. I'm sure you can find success using a different build, a melee one perhaps because their isn't a whole lot of open space to move around in this level, and a healing/Doc build might be useful but all in all not necessary. For the most part this deck should be what you run with but if you want to switch a few cards out with some you might see fitting better, then have at it. The most notable card not included imo is probably '
Admin Reload' which allows you to reload your weapon once you stow it. I didn't include it in the base deck cause with the attachments on my guns, I was able to reload my guns quick enough to never feel like I really needed this. I actually ended up finding this as a buyable card before I jumped down to reach the final boss so it turned out to be an unneeded but nice quality of life card. 2 more cards that could help are '
Phosphorous Tipped' and '
Pyro'. These cards will give an escalating chance to set targets on fire when firing continuously and boost your fire damage by 100% and give 3 temporary health for each kill with fire. I'm not sure how effective, if at all, they will be on the boss but if it is, then paired with an assault rifle or lmg it sounds like they could be really helpful. '
Fire in the Hole' (gain 20 temporary health and +25% sprint speed for 5 seconds) is one that might be helpful for when you end up using your pipe bombs.
GENERAL NOTES;
Because this is being done in Quickplay, you have the potential to join mid-game. If this happens to you, you won't have the option of picking your deck nor character until your whole team dies but it's honestly not a big deal, I was able to knock out around 75% of the bosses health before I died so it's entirely doable without them.
For my character I used JIM for his +25% reload speed and +10% team weakspot damage. If someone chooses him before you, HOLLY, HOFFMAN and WALKER are good alternatives as well as TALA if you have the dlc.
This map has the unique advantage that as soon as every player spawns in, high quality weapons and a ton of copper will spawn right in front of you which makes this level alot easier to get through. Cause this is only one level, and its mainly focused on the one boss, the deck you'll be using is heavily focused on weakspot damage. You'll move slower than molasses when shooting but paired with the attachments on your guns, you'll be so accurate, you'll feel like a laserbeam. And with 2 primary weapons, you'll be mainly focusing on the 4 tentactles, the main boss and if they get too close, then sometimes helping get rid of the mutations. For my 2 weapons, I ran with an M249 lmg and a M1A sniper. You can ofc switch up your weapons, play whatever suits you best. With all that copper, your team will get started on spending it. I prioritized pipe bombs, pain pills and shield first. Pipe bombs will help steer the ridden away from you when your chasing after the boss when it starts to move through the cave and pain meds will help give a boost of health in a quick pinch. If your team by now hasn't purchased a team upgrade, then you should go for offensive or support item upgrade and health. You can also look at attachments either to buy or some on the ground that your fellow players have swapped out.
GAMEPLAY NOTES;
When you leave the 'saferoom', you'll be greeted with a mini boss in the form of 4 tentacles. Their hits always land in the same area so find a spot they won't hit you (I found hopping up on the tires to the edge of the 1st tentacle you encounter as a really good spot to hunker down) and they wont pose much of a problem to you and your team but be careful cause after they die, they will continue to spew acid every 5 or so seconds until you kill all 4 tentacles which can get annoying very fast. After you killed all the tentacles, stock up on ammo and check and see what the buyable card is. Once you're down in the cave, focus all your attention on the bosses weakspot. The boss also has 4 tentacles but again, they always strike the same area so find a sweet spot (i prefer the right side), crouch down and unload until its health bar depletes and you defeat the 1st stage to this boss. In this area, ridden and tallboys will spawn in continuously until you defeat the 1st stage, but dont worry, if you have competent enough teammates, they'll take care of them but its best to spray a couple of bullets their way to help.
Once the boss enters its 2nd stage, it starts to move throughout the cave and you'll be given a timer. I dont know the exact time you're given but i'm certain its between 3.5-4 minutes, give or take. Don't worry about the timer though and just focus on keeping up with your teammates. Falling behind at this stage means you're all but guaranteeing your death and maybe even whatever poor soul who decided to come back and revive you. This is where your pipe bombs and pain meds come in handy. You may need to use a pipe bomb or 2 in order to get the riddens attention off you and your team, and put yours back onto the bosses weakspots. If you end up getting a chunk of your health taken than pop a pain pill and keep on moving. If all goes well, you will eventually end up killing the boss an popping this achievement.
This is only my 2nd ever guide so any notes on what to edit or how I can improve are very much appreciated.