As soon as the mission begins you will be getting shot at by cannons and gatling guns, head around this area towards the canoes, killing any enemies in your way. Once you are in the canoe be sure to kill the enemies that come out to shoot you, and when you get out of the canoe follow the path through the cave until you are inside the fort, now you will need to kill every enemy that you see and make your way up to the buildings with the cannons inside. When you reach the first cannon use it to destroy the four bunkers outside, and then head into the next room and use that cannon to destroy all of the boats in the river. Keep going until you get to the third cannon and use it to kill any enemies that appear, and once they are dead use it to blow a hole in the building across from you. Get off the cannon and fight your way to the last cannon, once you get to it take cover and throw some dynamite at it until it is destroyed (note: If you run out of dynamite there is some more down stairs). Once the final cannon is destroyed the achievement will unlock.