If when the target dies, your screen shakes it will mean you have got a headshot.

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. 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. When you get to the actual pit, you will see an opened gate. First thing you want to do is go to The Devils Pit in Velen. 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.

Witcher 3 overkill