Dante's Inferno

Heretics are first encountered in the 'Heresy' chapter which is just over the midway point of the game. These can be recognised easily because they look like female circus ringleaders and they have a purple orb glowing above their head. You cannot damage these with your Beatrice's Cross attack so you have to melee them enough and the orb will disappear. When the orb disappears, you can now attack them in any way you please. There are loads of these throughout the game so you shouldn't have trouble netting this achievement.