In Expedition mode, select "The Acropolis" chapter on the lowest difficulty in "Chapter Replay". You can also select "Chapter Replay Elite" to use your upgraded gear to make it easier. When the chapter begins, proceed down to the town to encounter some enemies after a short cutscene. Get the enemies attention by shooting at them, then fallback and take cover. Once the enemies are standing close together, shoot them with a poison arrow. If you fail to get the achievement, simply reload the chapter and try again.