Another easy trophy.
Just as the description says - beat enemies into an unconscious state. You will know they are unconscious, as opposed to tranqed or dead, by the three stars spinning around the heads.
In order to beat a guard into an unconscious state, you need to utilize the 1-2-3 combo with the circle button. Typically I would knock a guard out BEFORE doing the hold up for the dog tag - that way I could easily move the persons to a secluded location without worrying about another guard dropping by.
This can be done with multiple playthroughs on the same save.
Questions, corrections and neg vote justifications below.