This is a story achievement and can not be missed.
However I would like to add that when playing on Legendary, at the very end of the mission, you do not have to climb into the gun until the last second. Keyes will says when. I hit a checkpoint with only the 3 center health lit and tried at least a dozen times, unsuccessfully, to finish the game. However, when I ran and got a health pack and waited for Keyes to say the word, I jumped into the gun, almost died, but fired the shot. Hope this helps anyone that may have hit that same problem as I did!