This achievement is part of the Half Life 2 Episode 2 game within The Orange Box. This is unlocked during the game's main storyline and can not be missed.
You will unlock this achievement in Chapter 5 "Under the Radar".
You will get to a point in this mission where you are driving through a town and will run into a force field and right into an ambush. Go inside and take care of the ambush.
After taking out the ambush head back outside. You must now take out the power source to shut down the force field. Exit outside and enter into a building to your right. It has a power source and a turret. Take out the turret so Alex can power down the force fields.
Of course the fields come back up and your trapped. In one of the corners (the back right when looking at the power source) with a ladder and lock. Take out the lock, climb up the ladder and drop in on the power source.
When you get back to your car the achievement will unlock.