I just want to make another guide for this achievement as there were a few things that kept me from getting this to pop after 100's of kills.
First thing you want to do is go to The Devils Pit in Velen. This area has about 10-15 Human guards.
When you get to the actual pit, you will see an opened gate. You need to run in and agro as many of the guards as you can then head back just outside the gate and the guards will all pile up at the gate as if they can't get through. This makes it easy to get headshots.
Second thing is that when you are going for the headshots, make sure that the only damage you are doing to them is with the crossbow.(Not 100% confirmed but like I said, I killed hundreds prior to following this technique as before I was damaging them with fire first and got nowhere.)
The last thing which is quite IMPORTANT is to note that when you line all the guards up and you have damaged them enough to go for the headshot kill, make sure you have all the upgrades to slow time and stuff and watch carefully as when you kill them, to clarify whether you get a headshot kill or not depends on whether your screen shakes. If when the target dies, your screen shakes it will mean you have got a headshot. If your screen doesn't shake then it means that no matter how accurate you think you were, the headshot will not count.
(A good way to help this achievement out a bit is that when an enemy is close to death and you are ready to go for the headshot kill, you can use AXII to stun them still as AXII will not cause damage.)
Once you have the rhythm you simply need to kill all the enemies and when they are all dead, you need to fast travel to another city to them come back for them to respawn. (Fast Travel to Vizima and back as Vizima has the fastest loading time.)
I have linked this video below to where I got all this advice from and after following this guide exactly, the achievement popped with no problems.
Please Note - You can also clear up a bunch of other achievements in The Devils Pit as the enemies will continuously respawn after Fast Travelling away and coming back.