In order to get this achievement play through the entire game on Hard difficulty, and it will unlock at the ending credits.
The difficulties are also stackable, but there is no point playing the game on Hard, you should first do it on Casual, once you complete Casual you will unlock "Hard-Boiled" complete Hard-Boiled and you will get the achievement for Normal,Hard and Hard-Boiled, Lets face it no one wants to play this game more than they have too.
Some tips for attempting this on Hard mode or even Hard-Boiled.
1. Use cover- On the easier levels, you can just perform bullet dodging ballet without a care in the world. But try it on this setting and you'll be dead before you hit the floor. If you can when a new area starts find somewhere where the enemies come in through a bottleneck. If theirs cover too even better. Stay in cover till the enemy's run towards you. Lean out and blast them with ease.
2. Change the options of auto aim to high - You don't have to do this but it will make the game a tad easier.
3. Precision aim those bad guys. On most bosses, precision aim is your best friend and will allow you to do max damage with little danger.
If you find this solution to be unhelpful, please tell me why in a comment and I will update it the next time I'm online.