To earn this trophy, you need to kill 100 enemies with headshots. True headshots are always a 1 hit kill on all normal enemies. There is no in game tracker of how many headshots you've made.
Instead, just reach an area of the level where several waves of enemies come pouring in. You'll find at least 1 of these areas in nearly every level. If you want to also work towards-
than I suggest using semiauto pistols/ revolvers. (Dual shot and full auto pistols count as well, but are harder to score headshots with if you're aiming for both goals. If you are only focused on headshots, bolt action/ semiauto rifles work well for longer ranges, and pistols work great at close range.
If you don't reach your 100 kills during your run, just kill every enemy in an area that sends waves of enemies, until there is only one left, and let him kill you to reset the checkpoint, or kill him and then pause and reset the checkpoint. Your kills are saved regardless of dying. Repeat until the trophy pops.
Please vote if you found this helpful, and please comment if you have any additional insight or questions. I always reply within 24 hours. Check out my others guides on TT and TA! Feel free to add me as a friend if you need help on TT, TA, PSN, or XBL, all by username Danokitty.