You will probably get this just from normal play, early in the game.
Any time you are doing a mission that involves groups of enemies, such as hacking a ctOS center or invading a gang hideout, find cover while undetected and hack security cameras. Whenever the camera spots an enemy, it'll "mark" them so you'll be able to track their position after you finish with the cameras. You only need to "view" 100 enemies this way, with a camera, to get the trophy.
There is a skill that lets you craft an item to "ping" an area and tag all enemies. I am not sure whether or not that counts, because I got the trophy before getting that item. I will update the guide if anybody can confirm whether or not those "tags" count.