No More Room in Hell

As of update 1.12.3 THIS GUIDE DOESN'T WORK you have to complete all the levels by yourself, not sure about custom server plugins if they work but since now if you type anything related to insta win it will say in console that achievements are blocked for this session

More about update which patched this cheese: link

Basically all of those 40+ achievements can be done with ease, using console commands

For all maps use this command (Except the Northway map, which requires another command) (Make sure to start the game with nightmare difficulty before doing all those achievements)

(bind "key" "sv_cheats 1; extractallplayers; sv_cheats 0") (Just replace the word "key" in this bind to your key of preference, and just press it)

For changing maps without of need to exit to main screen/vote for map, just use this command (changelevel "name of level") where the "name of level" is either nmo or nms map (insert a map name without quotes)

For Northway map don't use the first command above, which is used for all maps use this command (bind "key" "sv_cheats 1; zomb_squirt_all; sv_cheats 0") (Just replace the word "key" in this bind to your key of preference, and just press it)

For achievements which require 6 or 8 people, just create a server and invite others, and use the first command to get the achievements done.

For the maps required of this achievement, the zombies can be a pain to deal with. But with the help of a secret gaming power called the Console, your worries for getting this achievement are over.

1. To be able to open the Console, go to the game's launch options (Properties → General → Set Launch Options),and type: -console

2. Create a local game of the map of your choice

3. Press "~" key, this will open the console

4. Once the console is open during the game, copy the following:

sv_cheats 1; ai_disable 1; sv_cheats 0
This will disable the zombie ai, preventing them from spawning and making the map zombie free

5. Complete the game as normal, repeat for all 3 required maps, and the achievement is now yours

This guide can be applied to just about every weapon in the game

1. Download and install this map: Outside Linkhttps://gamebanana.com/maps/190178

2. Open game, type this command in console before opening the map g_ragdoll_maxcount -1 (if you enter commands during the session, the lighting of the map will disappear for some odd reason

3. Pick up the abrasive saw on one of the tables and make your way to the blue box

4. Stand on the green platform (used to spawn child zombies) until lag becomes noticeable,then use the saw on the zombies until the achievement pops up

Subscribe this workshop map https://steamcommunity.com/sharedfiles/filedetails/?id=2644289626
Start the map and as soon as the round starts, grab the melon and go to the exit. You will get the achievement in 10 seconds.

Subscribe to the "nms_desensitization" map by Shizuru on the Steam Workshop for No More Room in Hell.
https://steamcommunity.com/sharedfiles/filedetails/?id=1637279982

Launch No More Room in Hell.

Create Server from main menu.

Under Server tab select Map "nms_desensitization."

Under Game tab set Difficulty to Nightmare. *If the achievement doesn't pop this is probably the step where a mistake was made.
*
Start game and press the orange button three times once it loads. Voila.