NOTE This will not work unless you have a macro working mouse or keyboard or something like that
You can put in a macro that can make a headshot without even moving your mouse, this is undetectable for your not injecting a dll file or something (i did not make this)
If you know the exact number of the pixel color of an enemy's face / head/hat then you can make your mouse move over there and shoot.
First you need to take screenshot of game with enemy visible in it.
Then you find the Pixel color of it using following script
Move your mouse over enemy's head in the screenshot. then press control z
MouseGetPos, MouseX, MouseY
PixelGetColor, color, %MouseX%, %MouseY%
MsgBox The color at the current cursor position is %color%.
Now you'll get messegebox containing pixelcolor's code
Note it down and then make new autohotkey scriptfile like this
Ass-uming that your screensize is 800X600
And the pixel color you are searching is 0x9d6346 (refer help file for more details, I'm only giving you a broad lay-out)
PixelSearch, Px, Py,0,600, 800,0, 0x9d6346, 3, Fast
Using above script you can press Left mousebutton and it'll move your mouse over enemy's head and then shoot.