The achievement describes an upgrade you need but the real name of the upgrade is Stun Effect in the staff menu. You must have at least 3 enemies near each other and shoot with the stun ammo. Hold and make sure you have stun selected () and press to fire the shot. If they enemies are close enough, they will all be stunned at once. An easy spot for this is in Chapter 8. After the fence opens and the mechs come to attack, they're all groups up running at you. Just fire a shot into the one in the middle.