Just to add to the above solution
Rather then targeting city guards the following solution involves Weepers who have no armour and lower health. The below method also works with Time Bend 1 but Time Bend 2 is highly recommended. An upgraded crossbow is also recommended. Load up Lady Boyles Party.
from Mission Select. Start the level purchasing Time bend 1 and 2
. Cross bridge and turn right past the Tallboy. You will soon come to a building filled with weepers. Make a save inside the building
Walk into a hallway filled with weepers, walk amongst them so they all rise up and come toward you. Activate Time Stop and Switch to your bow. Aim for their Heads
. Shoot them & move on, don't hesitate. You should be able to kill 5 within the allotted time.
Reload the save if it didnt work. Here is a quick 40 second clip of me doing it.