Let me preface this guide by saying: F*CK THIS ACHIEVEMENT! It is incredibly frustrating and a huge commitment to do, despite it being relatively simple in theory.
The basic idea for this achievement is to kill 40 enemies (they don't need to be just zombies) with a single activation of a corruption turret, the corruption turrets are controlled by you and insta- kill enemies. They only last about 25- 30 seconds, so you don't have a ton of time to do this. The problem with this is that you will never have an instant where 40 enemies are on the map at once, because the game caps the number of spawned zombies at around 25. So to get this you need to kill a full group of 25 zombies and then get an additional 15 or so to spawn in before the timer on the corruption turret runs out. This is much easier said than done. On earlier rounds, this is basically impossible because the zombies are not all running, and spawn in slower than at later rounds, and even so, on late rounds it is quite a challenge to get enough.
I'll detail how I did this, but first I'll explain how to unlock a corruption turret, if you already know how to unlock pac- a- punch skip this because a corruption turret becomes available every time you finish a generator, and can be purchased same place you activated the generator.
Activating a corruption turret
There are a total of 4 of these guys around the map, and as soon as you unlock one it is available for use, so you do not need to do all 4.
There are these things called corruption generators around the map, they are incredibly obvious and look kind of like gates to hell. If you don't know what they look at, spawn into the map and run to the other side of spawn and you'll see one, very hard to miss. The 4 locations are:
-Spawn
-Mob of the dead
-Verruckt
-Der eisendrahe
To activate any corruption generator, head to the back side of the pedestal type thing in the center and spend 500 points to activate it. This will begin a sequence where either keepers, furies, or both will spawn in and start attacking you. You need to survive for about 15 seconds before all these enemies die, whether that means shooting them or just running around avoiding them is up to you, this is very easy to do so you'll have no problem.
Once the generator is finished, you are awarded with 1000 points and can now buy the corruption turret here for 2000 points at the same location you activated the corruption turret.
Doing the corruption generator provides power to its respective island, which is necessary for getting anywhere on this map.
Making the rounds
Now, unfortunately, due to the nature of the zombies slower spawn rate on lower rounds, you will need to be at a quite high round to even attempt this. I would not even try it until at least round 30, I personally got it at round 38. If you're relatively inexperienced at zombies getting to these rounds can be quite difficult. Here are some tips to help you out.
Your perks can be largely up to you, but you will absolutely need juggernog and quick revive. Also, if you did not know, once you have 4 perks you can get a free random perk by doing this wall running thing at der eisendrache.
I recommend you have either a thundergun or apothican servant, and an upgraded brecci in terms of weaponry. To upgrade the thundergun is just like any other weapon, but the apothicon servent is different, and can be a little tricky, so you may want to just stick with the thundergun if you're not sure. You will also want lil arnies from the box, which you should spam out if you see a max ammo before you grab it.
The zombie shield is also very helpful as it prevents damage to your back, so you will want to build that as well, it it quite easy.
The main area you want to train at is kino der toten. This is actually a very good training spot and even relatively inexperienced players could manage.
So in between round 1 and 30 you want to:
-Have gotten 4 or 5 perks
-Obtained a brecci, thundergun/ apothicon sevrant, and lil arnies from the box
-Built and equipped the shield.
Obviously, none of this is required for the achievement, but considering it requires you get to such high rounds you want to make sure you're well equipped. You should easily amass a ton of points before you actually start this achievement so, feel free to grab any nukes you see as you want to get there as quickly as possible.
Actually getting the achievement
Once round 30 rolls around you can get started. Please note that this is a SOLO strategy, which is how I did it, it is actually made quite a bit easier with more people, and I'll explain how to do that later.
Once round 30 starts, first make sure its not a parasite round (obvious by the map changing colour). If it is, get through that (just spam out lil arnies, might as well because you get a free max ammo at the end). If it's not, you can get started. Train up the zombies in kino. If either a margwa or panzer spawns, kill them immediately (they do not spawn very often). As well, try and use your brecci to kill and furies you see, this is because the furies knock down zombies as they walk, which slows them down and cause them the respawn (you don't want this). Once all the zombies have spawned in, slowly make your way to der eisendrache from kino. You don't want to outrun the zombies, you want them to stay close by so they don't respawn in front of you.
Now you may be tempted to try using the der eisendrache turret, as it is right there. While this may technically work, I must have tried 30 odd times on this one and couldn't get it, I assume because the spawn rates are too slow in this area. Instead you want to go to the turret at mob of the dead, I got this my first try when I did it here (to be fair I was on quite a high round).
Head through the portal behind the der eisendrache turret, and wait in nacht for a few seconds until the zombies behind you start to go through the portal, now head to the other end of nacht and through the portal to mob of the dead. Again, wait until some zombies follow you through.
Now you don't want to jump on the turret right away, as some zombies will have respawned, so you want to group them all up again. I suggest you bring them over to the cafeteria and run them through there, allowing more to spawn in. This is a very treacherous area so you need to be careful, as a full group of zombies could easily overwhelm you. You will need to kind of bob and weave through recently spawned in zombies as you leave the cafeteria, which is quite tricky, if you get scared, don't hesitate to pull out your thunder gun or apothicon servant to save yourself, and just head back to kino to try again. Now once you leave the cafeteria, a full group should have spawned, so head over to the turret and buy it.
The turret is on a timer, not based on ammo so hold down the trigger the entire time. Kill the full group behind you and just spray at anything that moves. You are not invulnerable while on the turret, and zombies will spawn behind you, so every few seconds do a quick spin around to kill anything that might attack you. Shoot at every single zombie you see until you are kicked off and you just might get it, but understand that it is unlikely to happen your first time.
As soon as you are kicked off head back through the portal behind you and run back to kino, trying again next round. Solo, this is very difficult and requires a large amount of luck, if you have people to play with, it can be made much easier however.
Multiple people strategy
As with solo, you will want to reach about round 30 before trying this. The difference here is that the spawn rates increase quite a bit with more people, so it is much more feasible to get 40 in the limited time you have. The people not going for the achievement will want to buy widow's wine, as they are kind of acting as monkey bombs for the person doing the achievement, and they need to stay alive themselves.
Have everybody training zombies at different points on the map, but relatively close to the der eisendrache turret (where I recommend you try this). Bring your zombies to der eisendrache at around the same time. Now, once all the zombies are there, the guy going for the achievement needs to quickly jump on the turret and shoot everything he sees. The other guys should stand a few meters in front of him, so the zombies spawn in and attack them (try and kill the zombies before they kill the other players). As with the solo strat, make sure you watch your back as well.
I can't personally speak the effectiveness of this strategy, as I never tried it, but if you are in a game with decent zombies players, this should be much easier than solo.
No matter how you do it, this achievement is terrible. Quite a bit needs to go right for you to get it, and it takes a fair amount of skill. I personally spent 15+ hours just on this achievement trying it solo, so hopefully you have better luck then I do.