The Orange Box

This is story related. Once you have finished destroying the generators so you can get up on the roof, have gotten over to the other building, you'll head down to the bottom of that building and a guy will give you a rocket launcher. Then you'll have to fight a few Striders, which are huge incredibly tough bipedal machines. Then you'll be running through the city with striders popping up every now and again. The achievement soon after that.