This will be easily attained if you're running through the campaign on Normal or easier difficulties, especially if you focus on acquiring head shots in the beginning stages. Using the Rifle, try to only do charged shots as they are the only way besides using the saw blade weapon.
Try to wait until there are large groups of enemies, as the charged shot will go through multiple enemies, allowing you to both rack up head shots fast, as well as clear out groups quicker. I got this trophy by the end of Mission 5, so even at a casual pace you can get this by the end of the Campaign.