Deadly Days

The grindest of the achievements, for this specialization you need to win the game 15 times, each time will be harder than the last since all the "challenges" stack after each win.

There are 16 levels to this specialization but you only need to win level 15 and unlock level 16 to get this achievement.

The challenges will make your survivors gather less resources (scrap/food), make upgrades/repairs more costly while making zombies beefier,faster and hit harder.

Here are some tips from the top of my head:
The starting items/powers/survivors are random, if you start with a very awful combo, try to see if you can do the first few missions without trouble, if not, dont hesitate to create a new game while praying for good RNG.

After level 5 or 6, I stopped trying to get the factory win ending, although it takes more time, making the rocket space ship is the safest way to win.

Since you need a lot of tools for the rocket, I always prioritized hardware stores over anything else, garages often have 1 tool sometimes, it helps.

For food, I consistently was able to win with 3 or 4 survivors, building the greenhouse everytime to keep food consumption low.

If you are running out of food, you can go into a mission and if you are not able to get enough food, quit to menu and continue, you will be back at HQ and you can choose another mission to hopefully find the food you need, this trick also works if a survivor dies or it looks you are about to lose the run, pause the game, quit to menu.

Vending machines are always good to check, they often sell tools, food and the occasional legendary weapon, if you see a mission with a cluster of vending machines, go check them out.

The explosive bullets upgrade is godly on the gun that fires 10 bullets per second, unlock this gun first and if you dont like the upgrades, quit to menu and check the other custom guns.

Learn where the rocket parts usually spawn: In the middle of a corn field, in a grass area in a park, on a NASA van, in a fenced lonely garage, in a forklift on a street intersection, sometimes I just run around until I find the piece, quit to menu, continue and then scavenge towards the place I found the rocket part.

When you rescue a survivor, they almost always come with some food, same with hospitals, they spawn some food most of the time, keep this in minds when choosing missions.

Zombie Data
The poison spit from cloud zombies explodes in contact with fire, keep moving to avoid being inside the puddle in case it explodes on you.

The infested rat zombies are ok but the rats will eat you alive, specially if you dont have any AOE weapons like launchers, the rats can kill you super quick at night and are super dangerous.

The electric zombies hit as hard as the muscle zombies but they are fast too, luckily, they die quick if you have high damaging weapons, both of these are dangerous past threat level 25.

Flame zombies ignite poison puddles from spitters, its a nightmare fighting these at the same time, avoid.

Freeze zombies are the safest, just remember to escape before it gets too bad where they keep coming and wont let you move.

Tongue zombies are ok, they dont rope you that often and when they do, pause the game and airstrike them to escape, if you are careles though, you might lose a survivor to them if the night gets too fast and heavy and you get roped while trying to flee.

Thief zombies are awful at nightfall, after threat level 25, they will swarm you and steal everything you picked up during the mission, I just escape as soon as I see some scraps missing.

Best Items/powers
Invincibility is godly, upgraded it will give you 26 seconds of free kills, you usually can level up once or twice if the horde gets huge and fast, you can also use it to get back to the bus at nightfall even if you are far away, its the best power imo.

Speed Up or Hurry up and The Key item are great for looting quickly and get out before night, the power really shines when you get looting time penalties in the middle of the specialization.

News item will give you an extra missiont o pick up AND show you where the lootable places are.

Any of the funnels are great, to make your powers be used more times or to use the standard powers more often.

Coin on a string will let you use the potential strength of vending machines more often, super useful.

The Mallet will level up your weapons after each mission, making them do more damage, even custom weapons level up with it.

Vaccum Cleaner item will help you find parts, places way faster, super good item if you dont have the News.

The Recycler will help you with food, it says 50% but I felt it was higher like 70% chance of 2 foods.

The other powerups are obvious, more damage=good.

Wish you luck!