Your first encounter with Ceph Stalkers on the 2nd mission, Welcome to the Jungle. You might be able to do it the first time you encounter them, but it's a pretty small area. That might make it easier for you. I found it easier to do in the large area (with the suit jammer). After you take out the jammer, I found it really easier to track the stalkers with the binoculars. Following around while cloaked, put your visor down once you get behind, then stealth kill. They tend to go all around the area, so you may need to stalk the Stalker for a minute or so, but just keep trying and the kill will be yours in no time.