This is in Half-Life 2 Episode 1, Chapter 1.
To get this, as the debris falls toward the elevator, you can catch it with your supercharged Gravity gun and throw it to the side. Eventually there will be a stalker that keeps you from going any farther down. Shoot an energy ball with the Grav gun into the small room he's in, and it should kill him. Shoot another at the energy switch in the wall and the elevator will continue down. Soon you'll reach the bottom and Alyx will tell you to get off the elevator. If you listen, you live and get the achievement. If you don't, you die and restart.