For this Achievement will need to Kill help Emma kill Alexa. there are two separate ways to do this either let Emma do everything or speed the process up yourself. I find it easier to do the job yourself the reason for this is because there are so many factors that could go wrong with the AI on this mission.
For this achievement its best to start off in the Garden you do not need any weapons for this challenge. As soon as you start the mission run and turn on the radio. Wait for him to come over and subdue him drag his body into the garden next to the box don't take his clothes just yet. Now what you need to do its subdue the mansion staff member drag him to the same place as the photographer.
Now its time to start setting everything up run over the other side of the garden and follow the mansion along. Take a right so your keeping the mansion on your right hand side and in-front of you is the point where the items are delivered. On top of a box of wine is a crowbar take it and turn around going back the way you came. As you go back left into the garden you will see a fuse box on the wall crack into it and take out the fuse.
Once you have taken the fuse run over to where you started the mission you will see another open fuse box we need to place the fuse we took into this one. Turn around and go to the left hand side of the fountain you will see a wrench take this turn around and head into the greenhouse.
Go to the greenhouse BUT DO NOT break into it until the guard has walked around to the left of the greenhouse. Break in and go right you will see a book on the table scan it with your camera. Now fix the machine on the table with the wrench you picked up earlier. to the left on the machine is a flower pick it up and craft the poison.
We are finally ready, go and take the photographers clothing placing him into the box, Head over to the camera and take a picture of the maid. She will then call the family down. After we wait a little while the family will ask for a photo to be taken take it and when Alexa tells you she has had enough quickly run over and take the mansion staffs clothing and put him in the box also.
Quickly run inside to the sitting run there is a bar on the bar is a glass of what looks to be whiskey poison it. The final step is to go over to the other side of the bar and wait. WARNING: I had to wait 12 min 37 seconds before Alexa made her rounds and came to drink from the glass.
Once she does she will drop to the floor dead and the achievement will then pop you also can unlock the challenge Tasteless, Traceless if you haven't already.