This achievement is acquired by defeating the boss of the City area: C.O.P GORE. He is found in the room with the boss marker in the City area which is accessed by defeating B-DSS Knight.
This can be a particularly irritating fight if you do not have a ranged weapon as C.O.P GORE will spend the majority of his time away from the building roof. He has multiple attacks which can be difficult to deal with when stacked with one another. His most annoying attack, in my opinion, is one where he flies above the building and drops off two enemies. This is because you will have to fight them while also avoiding other incoming attacks. The boss can rapid fire a gun at you which can be dodged by running in a circle out of the line of fire, but this can be hindered by the presence of enemy robots. He can also fly over the rooftop and release explosives over his path. This attack can be easily evaded by just not being where he is flying over and rolling out of the way. Finally, he will launch a large but slow missile at you which deals a large amount of damage if it hits you.
When it comes to hurting C.O.P GORE, you can either: shoot at him whenever you want if you managed to pick up a gun, you can melee him whenever he flies over the roof (but this can be difficult as you can't always tell which direction he will fly in) or you can use the large missile he shoots against him. I prefer to use the missile as it doesn't require too many of them and doesn't take too long. Whenever he fires the large missile; be sure to attack it before it hits the ground or an enemy robot and it will be deflected back at him.
There is a large pole in the middle of the roof, which you can use to block some of the attacks he uses against you.
After defeating him, you will be take to the Space Elevator location - the final location - and you will receive this achievement.