This achievement is relatively easy to complete but the description is a bit unclear so here is a tutorial for anybody having any problems.

1) Load the mission Eviction.
2) Play through the mission until you enter the subway with your squad.
3) Try not too kill too many soldiers until the first explosion when the lights go off.
4) When this happens, kill as many German soldiers as possible.
5) The lights will go off two more times after this.
6) You will need to kill a total of ten soldiers during the black outs which can be quite difficult as it is dark and visibility is limited.
7) Once you kill ten soldiers during black outs the achievement will be awarded.

Here is a video tutorial: