To reliably unlock this trophy you will need Bend Time (level 2). There are a few special locations that you can kill 6 or more people instantly with a grenade or whale oil drum, but they are unreliable and luck based. You can also use the pile method detailed below and in the Hornet’s Nest trophy without Bend Time, but it may or may not unlock.
The following is a location that you can unlock the trophy:
Mission 4: The Royal Physician
At the start of the mission you will encounter 4 guards. If you sleep dart one of the guards and run around they will sound an alarm and summon two more guards to the area. This will put you at 6 total guards that you can kill. Sleep dart the rest of the guards, pick them up and place them in a pile. Now save your game in case you make a mistake. After saving you can use any method you see fit to kill the six of them in under one second.