This is pretty straightforward. There are lift scattered throughout the game. Especially in the Centro District. Once you come across one, get the attention of some nearby guards and have them chase you. For best success run straight towards the front of the lift as opposed to coming at it from the side. Once you get to the lift, just hit
to go up the lift and if timed right, stone blocks will fall onto a guard. Here is a video I did taken near the western corner of the Centro district and near the Vaticano District. This spot is nice because there are 2 lifts in the area. If you fail on one, there's another nearby to try again. In the video I killed one guard at one lift and then 4 guards at the 2nd lift.