For this trophy you need to get 20 kills while hanging on something for anyone who doesn't know what that means it's when you are hanging on some object you can climb.
This might come naturally to you when you playthrough the game but for me I never got single kill during my playthroughs so here's how I got it.
So during chapter 4 you will come across an area where there will be a big drop in the middle of it and in front of you there will be a doorway.At this point you should see guards walking passed just drop down and hang on the ledge in the middle you should be able to get 5 to 6 kills there every time after you get the kills just press the restart the last checkpoint option and just keep doing this till the trophies unlocks.
This video shows how to do it. Credit to KittyGotWet.
When I was going for this trophy I had the one shot tweak unlocked which made it easier you get that tweak for beating the game on crushing but you should do just fine without it.