You need to pick up and carry 5 guards to the same hay bale, using the B command next to their bodies after they have been defeated.
A great place to do this is during the invasion of San Gimignano. At one point in the mission, you will be required to kill (with help) 10 guards. Once you do, take the guards one by one down a single flight of stairs, turn left, then turn left again, and you'll see a cart with hay in it. Throw 5 guard bodies into the cart and the achievement will pop just after the 5th one.