For those worrying about earning this on Professional, there is an easy way! I only did a professional run on this game, so I had no normal save and I didn't fancy replaying the entire game to get to a decent level as suggested by other guides.
**NOTE** - This can be earnt on any level, below is how I did it. Now, just for information :
THE TARGET(S) YOU ARE ASSIGNED TO KILL DO NOT COUNT TOWARDS THIS TOTAL.
So, I restarted the game, selected cloud as storage (or usb) to ensure my professional save with all guns stayed intact, and started a new game on Normal. This puts you on the very first level, Asylum Aftermath.
You only start with a pistol, but thats fine, trust me. Check your map, all living people are highlighted. Enemies in yellow, neutrals in grey.
You will see lots of grey on this map. Go kill them all. They wont fight back. I got to the elevator with 18 kills.
Take the elevator to the second floor, grab the police outfit and the shotgun. Now, back to the first floor. Kill the nurse outside the life. 19.
Now, just use your shotgun to kill all the police. There is also a load of neutrals on here. Stop at 43 kills, as there are 4 outside you want near your escape car. I killed all the police on this floor, and most of the neutrals in the cells at the back. I stopped at 43, went outside, and killed the two snipers, the lone guard by the car, and the other patroling one. 47 kills.
Jump in the car. Done.
As this is on NORMAL, its virtually impossible to die/fail. It took me 6 minutes to get this.
The hardest part, is keeping a tally of where your up too.....but dont forget, on normal you have 7 saves, so abuse them if you want!
Id always suggest saving just before you exit the level, that way you can see your kills, and reload and add some more if you need too.
Any questions, just ask!