If you haven't gotten the Hornets' Nest trophy yet, stop and see my guide for that one first. It involves the same setup, but you should do Hornets' Nest first. If you are coming here after just getting Hornets' Nest, make sure you reload your save so you have 4 unconscious bodies instead of 4 dead bodies.
One easy spot to get this is Mission 4 (The Royal Physician). Start the level, go up the stairs, and there are four enemies in this little area. Use sleep darts or choking to incapacitate the four enemies. Don't let them activate the alarm or more enemies will show up. Move their unconscious bodies into a pile, out in the open street next to the alarm thing. Save your game now in case you mess up. Place 3 or 4 spring razor traps in the pile of unconscious bodies. Activate the nearby alarm and hide. Two enemies will come running up the street towards the pile of bodies. The two enemies should run up the street together to investigate the pile of bodies, and all six should get killed by the spring razors.
Sometimes the two enemies will be spread out, if that is the case just reload and hopefully next time they will run up the street side by side together.
Note that if you try to incapacitate more than 4 or 5 enemies to put them in a pile, bodies will start disappearing, so you can't make a pile of 6 unconscious bodies for this trophy.