The first thing you have to do is shoot the big spotlight on the gunship. Once you do that it flys down for a few seconds and comes back. You will see 3 ( i think ) blueish laser type scanners on the gunship and those are what you have to shoot. Once you take those out, the pilot drops the gunships blast shields. when the shields are down take whatever gun you have and unload on him. There are 3 weapon boxes along the wall so if you run out of ammo just grab a new gun. I did this on Officer difficulty first try without knowing what to so should be an easy achievement.