Description is fairly straightforward. Easiest place to do this might be The Complex in the evening. This only works when Egor is present (the default condition), so you can divert him to the party. If you divert him this area becomes inaccessible in the evening.
The Complex in the evening has, I believe, a total of only 5 Externalists. One will be lying on the ground near where you start, moaning and complaining and essentially helpless. The other four are staking out the mouth to the tunnel ahead, two at ground level behind boxes, one up on a platform, and one over on the right. They will all have their backs to you as you approach.
Once you've killed them all, you then need to work your way through the traps in the tunnels ahead, and kill Egor (remember he'll be using Aether to turn invisible, but he's not particularly powerful otherwise). After that escape and the achievement will unlock.
If Julia invades, you'll obviously need to kill her as well to escape.