This story-related Achievement is awarded near the end of Chapter 12 after you kill the two gunners that are barricaded behind sandbags inside El Capitol. Take cover behind the debris and lob grenades OVER the sandbags. Don't try to toss them through the little window openings, it's much harder that way. Just make sure they don't bounce off the wall and land outside the barricades. The Achievement will unlock after you progress forward through this room.
This can be earned on any difficulty level, so drop down to easy if you're having troubles, or just want to earn it more easily.