You get this on the 10th level Crash Site. Set the difficulty to recruit because you will take alot of enemy fire. Now just follow the mission and be sure to destroy all the buildings as you go, you'll move up the river in sections/villages be sure you got all the buildings,towers and mortars(if it has a yellow way point destroy it) before you move on. Around the second hostile village there's a low hanging rope bridge you need to destroy, shoot the edges touching the ground to get rid of it. The last village is a little tricky because there are a few buildings up higher than the rest just be sure to keep an eye out for them. Stop right before the boss ship area, Destroy all the buildings you can before you enter the area. So you don't have to while he's shooting at you. Once you have got them all enter the area and turn right, there are two raised buildings around the corner. After you get them circle the area to make sure you got everything (The two large bridge ends cannot be destroyed and don't count so don't bother) The achievement will pop once you Destroy the enemy ship as it is your final Target.