Go to Fristad Rock, Afternoon
To get Violent Delights head to Fristad Rock once you're far enough into the story to have access to the Pumping Station. If you sneak in and out you can also get Only the Guilty achievement at the same time.
Sneak into the Pumping Station
Fia and Charlie are in the pumping station together, alone. Sneak in and finish them with one bullet. I used Nexus slab but you should be able to line them up with a penetration sniper shot too.
If you're also going for Only the Guilty, just sneak out and exit the map.