Difficulty Achievements stack for this game--so it is possible to unlock this achievement by either completing the game under the "Normal" or the "Hard" difficulty setting.
The jump between normal and hard isn't very substantial so I actually recommend starting on "Hard" and only doing a "normal" playthrough if you're having serious problems.
Noteworthy: it takes a little while to get the hang of the controls and the acrobatic moves--they are not unintuitive but there's a certain "feel" to gunfights that is totally different from other games--stay on the move!
Always stay in acrobatic modes--sliding or jumping--and aim for the head. Specific instructions per level can be found in their respective achievement sections.