This is not as hard as it seems. Getting headshots is really important on crushing difficulty, so you need to get good at headshots regardless. If you are good enough you will be able to get this as you progress normally. If you are really having a hard time with it, wait until you have beaten the game once. Use the cheats to select the silenced pistol and you will be easily able to make it through some of the early chapters in the game by headshotting enemies using stealth and the silenced pistol will keep you from alerting the other enemies in the area.
Defeat 10 enemies with headshots in a row