Marksman is an achievement/trophy in Metro 2033 Redux, awarded for killing 15 humans with headshots at a range of over 30 metres. It is difficult to judge how far 30 metres actually is, so try and headshot enemies from as far away as you can. Moving back the way your ride came in Frontline and sniping the commissar while he is motivating the troops is a good start. The Hellbreath and Tihar are great weapons for this. This must be done in one playthrough but you can restart chapters to farm kills.