Using Mark & Execute, take down 6 enemies in under 14 seconds [Bronze / 10G]

Mission: Insurgent Stronghold
A few minutes after the "Insurgent Stronghold" mission starts you will have to go under a wooden bridge. At this point you must already have a full execution meter. While you are under that bridge, shoot it until the guards hear you. This will make them group up and that's exactly what you need. They have to be as close as possible together. There's also a checkpoint right there, so you can retry anytime.
When you reach the other side of the bridge you need to make your way through the building, go up the stairs, and wait at the window outside. Wait until the two guards inside the house and one from the outside are in your line of fire, mark them with R2 (PS3) / RB (X360) and use your execution to kill three enemies at once. Before shooting them, you should also throw some flashbangs (so the other enemies won't attack you). Now quickly kill an enemy with hand to hand combat to fill up your execution meter and mark & execute the remaining 3 enemies.
Also keep in mind that you have to play on Rookie difficulty and need to equip the strongest armor in the game. Otherwise, the enemies will kill you. Activate the Sonar vision of your goggles to spot enemies quickly.