This requires you to finish any district (by killing the Übercommander) without getting seen by anyone (at first I thought this meant not alerting any commanders, but it seems that's not the case). The easiest one I found was Roswell - Downtown
. Make sure you have the Constrictor Harness and just slowly walk forward until you get to the building the target is in (then enter using the harness). There are a few enemies walking around, but they're very easy to avoid. Aside from the commander, I only had to kill one clansman at the beginning and just avoid everything else. Just kill the commander, take the death card, return to base, and the achievement is yours.
EDIT: Heraizen has provided a video guide for those who want it:
EDIT 2: This achievement is known to be buggy. First of all, it appears you have to do this on your first time playing through a district (replaying it won't work). Secondly, it seems that this achievement can simply glitch out and not be obtainable on certain save files. The only surefire solution is to simply start a new save game.