Mikaeled245,551 Locked 03 Feb 2011 03 Feb 2011
I got this on the level "Yultec Ruins", which is not far into the game. Near the end of the level, on the bottom, is a series of bridges with explosive barrels next to them and patrolling soldiers. I recorded it below. You only see two of the people in the video, but I went back and replayed the level to make sure and there are indeed four people here. If you don't see four people (i.e. someone patrolled to far away), go even further to the right and there will be another bridge and a red barrel (this is where I get the achievement in the video), the people on the first bridge will retreat among the others at the second bridge if you destroy the barrel without killing them, since they need cover.
If you mess up you may have to restart the level as the bridges stay broken after you lose a life.