In "Under the Radar" you will eventually come to a van that has run into a building. Go through the van to pick up some grenades (if you need them) and you'll see a large metal plate leaning against some support beams. Walk behind the metal plate and use the gravity gun to push it down onto the ground. Circle back to the front of it where the garage floor sinks underneath the plate, through a grenade underneath it, and quickly move on top of the plate. When the grenade goes off it will propel you upward to unlock this achievements.