I may have found the easiest solution to grind this achievement. It involves a glitch/bug.
In level 2 progress until you find the first hiding place like shown in the video. It is divided in half by a wall. You need to whistle and the soldier will trigger an alarm. The glitch is that he can not pull you out of the hiding place if you place yourself on the right side of the wall and sometimes he will trigger the alarm 3-4 times in a row without any interaction from you which that will make this solution probably the fastest way to grind it.
Enjoy