Max out the pedestrian slider on the "Performance" tab.
Get a bunch of guns from the Police Station.
You need to do a killing spree. Go to a place where you can see at least 10 npcs and kill them as fast as possible, keep following them while they try to escape and kill more of them. With every kill, the Dude will say a line such as "one for the...", he will repeat it a few times until you are done.
Don´t stop killing until you get it. Keep up the pace.
300 machine gun rounds will do the job.