This Achievement is won in Half Life 2 Episode 1. It's story related so your guaranteed it as well. All you need is complete the stage where you are trying to contain the Citadel core. Which if you remember from Half Life 2, you made unstable with all your interfering and whatnot. Make you way up through the level, it's fairly linear you'll not get lost throughout. If at the beginning you are having trouble getting the bridges to stay active then kill the stalkers best if you use the Pulse Rifle it's really the only thing with that kind of accuracy at that range. It will unlock I think when you meet back up with Alex, either that or when you physically containg the core I can't remember.