Call of Juarez: Bound in Blood

Description is quite explicit. Just aim carefully at the head of your opponents before shooting. A rifle makes it more simple from afar. When you shoot your 99th head, you'll unlock the achievement.

Note that the statistics in the main menu are so well done that they indicate where you are at with achievements counting your results.

Head shots are most easily obtained by using rifles, ranger pistols and volcanic pistols and better quality versions of these will help further.

Your long-range accuracy is also improved by playing as Thomas. If you insist on playing as Ray, note that you can't use LT to aim if you are using two pistols.

Take your time and mark your shots, always aim using LT - many enemies in this game like to stand still to shoot making them easy targets.

For me, this achievement unlocked on Chapter V, but it is cumulative across all playthroughs. Check the achievements menu for your progress if you are unsure.