A kill may not always count as a head shot, even if you fire at the enemy point blank in the face. Humans, dwarfs and elves will count as valid targets. Killing drowners will not count. You can damage your target with other weapons, but the killing blow must be a headshot with a crossbow bolt. A successful headshot is indicated by the screen shaking at the moment of death, with yellow text on the left side confirming a critical hit was made. Consider farming kills at the Devil's Pit in Velen. Lure enemies to the gate. They will not move past the gate if you do not get too close. Then, begin shooting them. To stop the enemy from moving around use the Axii sign then fire a free aim kill shot. After killing all the enemies, fast travel to another location, then return to respawn more targets.