Just a note that
will stack against this.
It's also confirmed as of June 12 2015 that even after the addition of the Nightmare mode update after 9 Dec 2014, that completing the game on Hard will still pop this achievement. (Thanks I Slaine I, and TheRedButterfly)
Hard Mode you'll find that enemies are generally more aware of your presence and depending on your strategy (stealth or kill everything that moves), will be harder to kill.
Save often and you should be fine - there are plenty of save points. Just be prepared for some frustration - missions 10, 14 and 16 are particularly long.