Actually killing alone is rather easy. You just place yourself in a good position and kill one. Then all the others will follow and you just pop their heads one after another. Doing it silently without triggering a single even suspicious trigger is a bit more challenging. I was often staring at the guards movement for several seconds or maybe even minutes. Then i went for a kill, dragged the body out of the view of others, disabled cameras by hacking systems and cleaning some more. Sometimes (most of the time) i just sneak past. It takes much longer but at least for me this just feels more interesting.