If you haven't gotten this trophy by the end of your play-through; load up chapter 8.
Continue through the chapter until you get to the point where Sully drops you down a with a bucket, there are 3 people that can be stealth killed within 30 seconds of each other.
Take out the three with stealth then reload the checkpoint until you hit 50.
P.S. this is also the chapter to get 30 with the dragon sniper later in the chapter.