In Sequence 3 you will come into contact with the courtesans, opening up a line of tasks that you can do with them.
The first one involves tailing a courtesan who is hanging around with a ne-er do well and killing that ne-er do well from a hiding place (a well, if I recall correctly).
The good thing about this quest or task is that it will take you directly to a spot on the map where you are guaranteed to find a person with a broom sweeping.
After being given the task, go to the spot on the map where the courtesan is. It will be inside an outdoor area surrounded by buildings. As you approach the area, don't go through the main entrance (as the courtesan will spot you and the task will be failed) but instead climb up one of the surrounding buildings nearby.
When you climb up, look around. On one of the buildings - located to the left hand side of the main entrance to the outdoor square (if you are looking inside the area) will be a man using a broom to sweep.
Locate this man, climb to his building and grab him. He'll drop his broom which you are then able to pick up. Walk down the stairs and look for a pair of guards that are nearby. Simply wander up to one and belt the crap out of him with the broom until he keels over.
The achievement will pop once you kill him.