When you are at a point that you can only use non-lethal attacks, make sure that you have the SC-20K and at least one Sticky Cam. Instead of waiting to sneak up on a person, shoot them in the head with the Sticky-Cam. This knocks them out without using 'lethal methods'. Aiming high is the key to knocking the person out because the Sticky Cams have a short range. After the attack you can pick your cam up and use it again.