This achievement is obtained during the Chapter 4 level at Vimy's Ridge where you play as Freddie aiding the Canadian soldiers. The level will begin with you shooting one of the cannons and destroying a German bunker, and then you'll be running to the right while avoiding machine gun fire.
After this short section, you'll reach a second cannon behind a dirt mound with a partially destroyed house in the distance and a German flag flying next to it. Use the wheel to the left of the cannon to adjust the cannon to 50 degrees (halfway between the 60 degree and 40 degree tick marks). Pull the lever, and the cannon will fire, taking out the flag and earning you this achievement.