For this achievement, you'll have to quite a bit of roof running using the "out of map" method at the beginning of the level. It is a lot to explain using only text, so the video is recommended. You'll have to use the light fixture in the doorway to crouch-jump onto the roofs above. From there, you'll have to navigate the rooftops across the map, onto a bridge, where you must grenade jump onto another building. From there continue forward and you'll notice the rooftop of one of the buildings has a hole in it, where you can drop down and find the "Rex Sword" with infinite ammo.