This is easily done in Mission 6, "The Beacon" (Chapter 3). This was recorded on Easy difficulty. Simply use your arc weapon to activate a stalker rack for the "Pied Piper" achievement / trophy. Keep doing this on stalker racks until you have 6 robots following you at once for the "Robot Army" achievement / trophy guide.
Here is a video guide with English commentary for this achievement!
Here is a playlist of guides for this game!
https://www.youtube.com/playlist?list=PL-HeW6XkKdUL1_8Q01lMy...