Spoiler sections are just to break up the text for easy navigation and sectioning, I do not believe anything in this guide is a real spoiler.I was adding a comment to the other guide here but it ended up being longer then their guide so thought id add it as its own guide (sorry
):
As the previous guide said a good place to grind this is
"Devil Pit" in Velen. Fast travelling to Vezima and back to respawn the bad dudes.
WARNING! This will stop working if you loot the chests in the devil pit as it will no longer be an active bandit camp!
FYI, Non Humans is the collective name used in game for Elves and Dwarfs so this is
not referring to Monsters.
Use strong bolt that don't have a side effect*** Spoiler - click to reveal ***I found the best and cheapest bolts for this were the crafted "precision" bolts. You can buy all the feathers you need from the master weapon smith by meditating for an hour to replenish his stock so you should be able to get hundreds of bolts really easily. Cant remember where you can learn the schematic though sorry.
Do it on easy!*** Spoiler - click to reveal ***The difficulty makes a huge difference for this one, I found on easy that a crit would kill them but a normal hit wouldn't so you get two attempts per kill to get that all important crit.
Harder settings you wont be able to get one shot kills as easily so would need to drain some health. Which is difficult to do without killing them if you are high level (I found a good way to do this but its easier just to do it on easy).
NB. If you are going for the "difficulty" trophies then keep this one till after you beat the game to prevent loss of progress towards them.
Bow crit is King*** Spoiler - click to reveal ***On easy and at end game you shouldn't be struggling to one shot them but "Steady Shot" in the general talent tree gives you 25% more bow damage for one skill point so its worth picking up.
But its only "Anatomical Knowledge" from the combat tree that really makes a difference on this trophy. "Lightning Reflexes" can help to line up the shot on moving targets.
Wearing the Assassins gloves for an extra 5% crit can help too!
Devil pit gate is glitched*** Spoiler - click to reveal ***The bandits can leave the camp when you first open the gate, I think its a bug that when you open the gate then reload the area the AI thinks the gate is closed but can't open it because it is open 
This means you can retreat to the entrance to the camp if needed and aim without being attacked. I doubt they will ever patch this but if they do, sorry.
Not all head shots are head shots*** Spoiler - click to reveal ***There are lots of debates about what actually counts as a head shot, some say the screen shakes others say its just a crit message in the combat logs.
Regardless of what it actually is I felt like I made several hundred valid headshots before this popped for me, so be ready for the long grind 