For a shooter that's shooting right handed on a left side barricade?
Stand far enough behind the barricade, so as to have the rifle or shotgun mounted in your right shoulder pocket. Meanwhile...have your left foot planted far forward, with a big knee flex. Have your right foot planted far rearward, with maybe your right knee locked. Lean forward, so as to have your weight on the ball of your left foot.
Lean you body to the left of the barricade...aim and take the shot.
This position is best demonstrated by Matt Burkett's DVD: Shotgun Mastery