As previously stated, this is both cumulative and simple to accomplish over the course of the campaign.
I simply wanted to add that I found a very easy spot to rack up a pile of headshots in the event that you aren't the type to use wingsticks on a regular basis.
On the "Mutant Expansion" mission (Subway Town, disc 2), there is an objective to find a TNT cache. Upon finding this, you will be ambushed in a small, enclosed area. Equip wingsticks and position yourself in any corner of this room. Aim your target toward the head of each mutant in front of you and let 'er rip with LB. There are plenty of mutants to one-shot in the face.