I found a nice way to kill Stalkers at your leisure in Chapter 6.
You enter a warehouse room, where you trigger about 6 or so Stalkers halfway across the room. As soon as you hear the Stalkers make noise announcing their arrival, run as fast as you can to the stairway you entered from. The doorway from the stairs to the warehouse room is a transition point in the game, and if you cross to the stairs, the Stalkers become disinterested and wander back into the warehouse.
Simply cross into the warehouse, wait for the Stalkers to run around and take a hidden position, and headshot them a couple of times when they peek out. As soon as you shoot once, back up through the doorway, and they will not run at you and attack (or will even turn around if they were already running your way). You can pick them off one by one at your leisure by repeating the in-shoot-out pattern.
Obviously, for this achievement, you'll need the Seeker Rifle, you'll need to aim (LT) and select the alt-mode (RB) before firing the killing shot.