Daymare: 1998

The first melted man is encountered after using the computer in chapter 2 to self destruct the archive. You will then head back to the parking garage and be treated to a cut scene introducing the enemy. Simply run up to the enemy and punch it. DO NOT ATTEMPT THIS ACHIEVEMNT ON A NO DEATH RUN AS YOU WILL LIKELY DIE FROM IT EVEN ON THE ROOKIE DIFFICULTY. The achievement did not give the notification for me when I did it, however it did unlock it. Below is a video containing the area mentioned above at 10:47.