When you unlock this achievement will vary depending upon how you play the game (e.g. Street Crimes, how many enemies your partner gets, etc.). However, rest assured you will unlock this during your first play through. There are a number of large shootouts towards the end. I played through the game pretty normally and it unlocked during the final Vice case called “Manifest Destiny”.
To track your progress, pause the game and scroll down to “Statistics”. The number of enemies you have shot is under “Gunfighting”. Find it quick by hitting
twice. Enemies shot dead is first number.