This achievement is part of the Half Life 2, Episode 1 game. This is a main storyline achievement and will unlock as you progress through the game.
This achievement will unlock during the last chapter in Episode 1 (Chapter 5, "Exit 17") at the very end of that chapter.
You will start the chapter off with the task of transporting groups of personnel to an extraction point. You will unlock an achievement if no one dies while transporting these personnel. Please see the following achievement for more information on how to get past this point in the game:
After you're finished getting everyone to the extraction point you will have to follow Alex outside, encounter a strider attack, get to the top of some large shipping containers. As you traverse on top of the shipping containers you will be attacked by a strider but just take cover and move forward. You will run into a stockpile of RPG ammo to take him out with plenty of cover.
After taking out the strider you are pretty much finished. Meet back up with Alex and get on the train with her. During the cut scene you will unlock this achievement along with "The One Free Bullet" achievement if you only fired one bullet. Check this out for more information:
The The One Free Bullet achievement in The Orange Box worth 104 pointsBeat Episode One firing exactly one bullet. Grenade, crowbar, rocket, and Gravgun kills are okay!
Enjoy and good luck!