During Mission 3: No Man’s Land, you will take control of Riley a few times. During the first section, you will need to take out at 4 enemies as part of a short tutorial. Then you will enter a stealth section where you need to use Riley to scope out enemies, and reach the end of the area. When you are not in control of Riley you can press when aiming at an enemy to get Riley to attack them. After killing 10 total enemies, the achievement will unlock, and you’re free to end this section of the level, by progressing into the next area.