This achievement can be somewhat annoying as you don't have a tracker but it will unlock as soon as you kill the last enemy.
First of all when you start the level, make a 180 turn because two enemies are behind you at the start. Once these a dealt with go to the rail car.
Now I would advise to just go loud here. It will draw the enemies towards you instead of going trough this stealthily and not finding the last few soldiers.
- Make sure that you kill the two wounded soldiers on the beds in de back of the platform next to the one you start on.
- Also make sure to go down the stairs where the Nazi's are where the Soviet captives are held. Make sure to throw a grenade into the cell so that it kills all captives and the commander.
- There is also a trolley right after this which you also need to blow up.
As pointed out before, if the achievement doesn't pop just keep looking as it will unlock upon killing your last enemy.
This video illustrates the level but it is pretty long so I would advise you to only use it if you are stuck: