Stand here and do some 'spray and pray': Fire the Heavy Machine Gun continuously whilst tapping the up button, killing every enemy in a 90 degree radius. Continue onward, killing a few more crustaceans, and stop when the big door opens. Continue forward, killing the next to crustaceans and freeing the prisoner. When you get to the building that spawn the crustacean monsters, through grenades until it explodes. As soon as you land in the water, start running and constantly shooting. I recommend playing the first mission on Easy difficulty. You need to clear a mission without dying to get this achievement.
