I heard that you can still kill people on the list even if they're not the assigned target for the Sacrament. I tried this today in the Sewer Tenement Sacrament. I need Dolwinora for the achievement, but she wasn't the target. I figured I could just find out her location from youtube videos (appeared to be the large room in the middle on the north side of the map, lower part of that room on the western side), and kill her there. I went there, but she was nowhere to be found. When I went to the place I saw her in the youtube video, she was replaced by a random NPC with one of those generic names. Killing this person did not tick Dolwinora off for the achievement.
So...did I do something wrong? Do I have to do things in a specific way to kill people who aren't my targets? Or is it just not possible to get someone if they're not the target you're given?
Guildmaster of Spectral Liberty - Xbox One - European Megaserver