You fought 5 enemies at once and none of them survived
You cannot unlock this trophy unless you have alerted five or more enemies and have them actively engaged, at the same time.
At the start of Mission 2: High Overseer Campbell, you will pass five soldiers on a bridge that dumping bodies into the ocean, and guarding the entrance to the city. Three of them are always up top and two of them roam underneath the bridge. Run around the bridge until you have all five of their attention. You will have to run to the end of the bridge near the lighthouse to get a few of them, so don’t forget them. After all five are pursuing you run in front of the Wall of Light and proceed to kill them.
It is easiest to kill them one by one with a pistol so be sure to have some ammo on you. If you are out of ammo, counter killing them (blocking right as they swing their sword with , then attacking with yours ) is the next best way. Once all five are dead the trophy will unlock.
An alternate easy location is during the second Granny Rag's optional mission. As you enter the Distillery, descend the stairs and a thug will warn you to back off. Hang around him for a few seconds and he will attack, along with 5 other thugs. Kill them all using whatever means necessary to unlock the trophy.
Another location is at the start of Mission 4: The Royal Physician. Walk up and kill/sleep art one of the guards, which will result in them sounding an alarm. Two more guards will then come running from across the bridge, giving you a total of 5 enemies to kill. If you used Sleep Dart on the guard instead of killing him you can also unlock Tempest and Hornet’s Nest if you haven’t already.
To see a walkthrough of the the method being performed, please see the video below: