This looks like a nice little challenge for any Assassin's Creed: Revelations players out there. To get the 'Lightning Strike' achievement, you need to take out five guards in five seconds or less using your hidden blades.
I know I wouldn't have been able to get this one without a guide like the one below. YouTuber OmeGigaSavior does a great job of illustrating a very effective strategy to taking out those pesky guards. If you manage to get the achievement, you'll snag a bronze trophy on PlayStation 3, or 20G on Xbox 360.