If you do not want to fail the mission while going for this achievement and having missed the opportunity when your squad wants to cross the street, there is a good opportunity near the end of the mission.
When you are told to guard your target in the mall, clear out the first two waves of enemies and move your location with the target. Clear the next waves until you come to an autosave and the new mission to get your target out of the mall. For starting the script, you have to go into the little room and get close to your target.
Immediately when your partner begins to carry the target, leave the room, look into the direction where the first guard-position was, get into cover and equip the infrared sniper rifle. Get into infrared-zoom and look to the old guardlocation. Another enemy wave will get in at your old spot and there are two or three occasions, where two or more enemies will run directly behind each other to your position.
If you fail, get yourself dying (if you took out every enemy inside, simple run out off the mall and stand uncovered on the plazza) and the game will set you back right to the point before your partner starts carrying the target.