Assassin's Creed Unity - Guillotined - Kill an enemy with a lift counterweight. [Bronze / 20G]

Lifts can be found all around the open world. Running towards them will automatically make you cut the rope that activates the counterweight. If an enemy is standing below the lift it will kill him and unlock the trophy / achievement for you. Simply get detected, look for such a lift, and wait for the enemies to catch up. Lure them below the lift and then cut the rope.
The place in this video works great because it has two lifts in one place and there are always two guards spawning nearby. So if it doesn't work on the first try you still have another lift in the same spot. You can also fast travel to the other end of the map and then come back to reset the enemies and lifts.