It is best to get this Achievement in Chapter 4 once you enter the sewer area you will face alot of enemies, firstly disarm the traps so they won't kill the enemies and then equip your crossbow with Flash Bolts. Now round up the enemies and get them in a group and fire a bolt at them, then quickly run at them at press
and you should get it.
Flash Bolts are found in C3 in the Watch Tower and the Crossbow is in a building next to the watch tower.