Enforcers are shown on the map as a skull symbol. For this trophy you should take out all of the skull symbols before continuing with the racket. You will get a warning that all content will be missed if you continue, so you will know if you are about to miss this. Generally it makes sense to go for this one on every racket as any enforcers you leave behind will chase you down and try to kill you at random points until you kill them.
Note though that you should leave all enforcers at least 1 time for the trophy: