I cant be sure if this was the only way, but when I played normal possession did not work for this achievement. Might work later on, so give it a try.
ONLY way to get it was gettting possessed by timeout.
So just do the embalments, take your time. Try to witness as many events as possible so your stress level goes up.
Eventually, in my case, I decided who was the demon, created the Mark, and just waited in the embalment room.
Once the lights start flickering like crazy and you cant turn the lights off. Try walking around. I got possessed while in the corridor between embalment and fridge rooms.
You will get the normal game over ending, and the achievement.