Unless you consciously go for headshots during your two playthroughs this will most likely be the last trophy you earn. Two normal playthoughs will net you about 150 headshots.
If you still need this trophy after your two playthroughs load up the 1st mission (Patriot – At Scene) on rookie and take your time going through the mission only focusing on headshots. This is should be very simple because your character is now fully maxed out with the incredible M260 Heavy Machine Gun. The gun holds 240 rounds per clip and has ZERO recoil. You should rack up about 90 headshots in around 10 minutes. Repeat until you get the trophy.
You can check the progress of this trophy at any time from your profile stats in the main menu.