Simply kill 5 enemies without being detected. You can do this on a few maps. Easiest would probably be Chapter 13. There is a section that starts where you see a keeper walk into a flaming room.
There will be an enemy down the hall to the left. Wait for him to turn around, take care of the trap and sneak kill. At the intersection, you will see a fake Ruvik heading your way. Stay hidden and watch him. He will turn into the room near you and once he enters follow him but do not get too close. He seems to notice you when you get close while he moves. Your moment to strike is when he stops and looks side to side. Stay on the other end of the wall near the door after he enters the room and wait for him to stop. Then sneak and kill him
In the next room which is kind of the same room since there is no wall. There is another haunted by the door outside. There should be a bottle you can use to your advantage but you should be able to sneak up on him without it. He moves back and forth and the distance is pretty short. You will need to be quick with him.
Your 4th victim will be in the next room. Hang by the exit to the balcony area but dont sit outside. A haunted will appear outside eventually on the balcony next door. Wait for him to turn back inside and then go for the kill.
After you kill him you can slowly move forward. You will see a bomb on the wall. You can move a bit past it but do not get too close to the hole in the wall to your right. Wait a little distance away and another fake Ruvik will appear. He will walk out and then turn to his right and stop. You can then go for your 5th victim. There are still another two haunted around. One is somewhere down the hall and the other is in the room fake Ruvik #2 came out of.