Guillotined
Kill an enemy with a lift counterweight.

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You can do this at any point in the game, but it is easier if you have smoke or stun bombs as you can make an enemy stand still underneath the counterweight. What you need to do is lead an enemy just below one, then use the lift so the box falls down onto the enemy. It might take a few tries as the box is small and the enemy needs to be in the perfect spot. I did it at the "Palais Bourbon" and there are lot's of enemies here and quite a few lifts. Any location with lifts works well though; you just need to lead enemies to them. Here is a visual walkthrough if you need further assistance on the location.