There are six achievements that require certain numbers to be reached - 200\2000 killed enemies, 50\200 thrown grenades and 50\200 saved POWs. Unfortunately Steam achievements tab doesn't show the progress for these achievements so I figured I need to find a tracker for these numbers.
User game statistics is saved in a file UserDefault.xml
The file is located at C:\Users\%username%\AppData\Local\mslugx . Open the file and look for the corresponding lines - see below.

Nurse - 50 saved POWs
King of Recovery - 200 saved POWs
Look for the line <powsRescued>(number)</powsRescued>

Grenade Lover - throw 50 grenades
Grenade King - throw 200 grenades
Look for the line <grenadesThrown>(number)</grenadesThrown>

200 Tombs - kill 200 enemies
2000 Tombs - kill 2000 enemies (that was hard to figure, right?)
Look for the line <soldiersKilled>(number)</soldiersKilled>

All in all, it should take 4 complete solo walkthroughs to get all the achievements. I haven't yet checked which levels are the most POW- and enemy-rich and are better for farming so if you have any idea please write it in comments and I'll add it to the guide with thanks.
P.S. I strongly recommend against modifying the numbers in xml file as it isn't fair play most definitely.