Extremely easy, first achievement you unlock. To start off, defend the destroyed church with your Lieutenant and some other soldiers. There is a staircase with a sniper rifle laying on it behind where you start out at. The ammo stash here is also unlimited, so grab it whenever you like. After defending the church, you get a cutscene with the General. After that is over, head out and do a U-Turn to the left of the building, and follow the path to a small encampment of Union soldiers. Defeat them, and go right from the way you came. You'll find a huge Union site. Jump on either cannon to your right at the end of the path and shoot the tower, which will be at the path's 10 o'clock (look right for the tall wood structure). Feel free to knock out some Union boys with the cannon down below. Next, move up through the camp until you find more cannons, and use these to shoot a tower straight in front of you. Shoot it, then head towards it and turn right. Follow this path and you'll beat the mission, as well as grabbing the achievement.