Outbreak

So, it appears that the update that happened a while back that fixed the old bug now is bugged positively. It seems people are unlocking this by completing the campaign only. While I can only attest to my own experience pre-update, you may/may not now have an easier time unlocking this

This achievement appears to be a bit buggy. See the comments for the full hypothesis. Basically it appears that the counter either resets after a session or saved data is not recorded properly after finishing a level. I had no problem with it but I also never shut the game down/ did not switch games. I would recommend at this time to do all stages in one sitting. This should take about 2-3 hours.

I would highly recommend you play these in this order:
Chasmal, Miasma, Enkindle and last, Outbreak.

General tips I have are:
-reload off screen
-hide behind your AI
-spiders are the worst; put some distance between them
-use lots of baricades
-check the map
-Play on normal with the AI.
-I also recommend using Quies because of the large inventory.
-If the multiplayer is ever released try that out.

If you get stuck here are the passwords:
Chasmal-2244
Enkindle-1964 and 86421 (thanks Vitiated1)
Outbreak-27591, 99991, 237732, 224466

Enkindle might give you a little problem at the very end when you are supposed to use the exit. Your AI might not enter the room. You can either try to get it killed or eventually it will enter the room.

Outbreak.......what a long level. No, using the lockpick character Alendra, will not save you any time. Leave any unused supplies and weapons in the central lobby.
The card keys do not disappear for this stage after you use them so, when do you discard them? It is also hard to tell where you need to go sometimes because you can not zoom in and out of the map(steam version you can/update coming please?). All of the directions are in reference to Central Lobby/start. So the below should help you:

SW corner of Central Lobby>white card key
S Hospital Entrance use white card 2x discard, 27591>red key
Central optional to the west use the red key to open a door>password
N Data Processing use red card 1x discard, 99991>gold key
W Auditorium gold key>yellow card
E Morgue use yellow card 1x discard, 237732> orange card
NNE Primary Triage use orange card 1x discard, 224466> blue card
Central Lobby + Data Processing optional use blue card for ammo, weapons and supplies
NE Laboratory use blue card 1x discard>silver key
S Garage use silver key, kill tyrant>floppy disc

...try not to step on anyone's laptop