Easy "Clever Girl!" achievement
In Mission 2: Welcome To The Jungle, you will encounter Ceph Stalkers for the first time. Shortly after entering the underground area, you will enter a room with tall grass, and a cutscene will begin to play. Go down the stairs, and stay away from the grass. Activate the cloak ability, and stand still until a Ceph Stalker is close to you. Then, quietly sneak up from behind, and perform a stealth kill to get the "Clever Girl!" achievement.