To get this, simply kill Cohen and take a photo of his body.
I recommend doing this in Olympus Heights (shown above), and not in Fort Frolic where you first meet. I'm not sure if you can enter his room in Olympus Heights if you kill him in Fort Frolic, which is necessary for another achievement.
Another option may be to save, kill him and get the achievement, then load and continue on. Credits to theicemansteameth