Of the Resident Evil 4 achievements in the remake, Real Deadeye is one of the more difficult ones to unlock. You will need to complete each of the 12 shooting range challenges to reveal the bonus condition for each. Once you know the condition, you will need to meet the criteria to unlock the bonus round (for all except 4-C), giving you a chance to rack up a higher score, boosting it from an A rank to S. The bonus conditions are usually for killing all pirates, killing them in a specific amount of time, or killing them within a specific number of shots.
Shooting ranges are found by taking the elevator next to the merchant in Chapters 3, 9, 11, 14, and 15. A new set of 3 challenges will unlock when you reach the range in the first 4 chapters listed. Chapter 15 is there to give you a final chance to complete the challenges before the end of the game. You can save all challenges until you reach either Chapter 14 or 15.