This achievement is located in Chapter 5 just after you see the helicopter. You will kill one guard standing on a rooftop across from you and then drop down next to a blue car that looks like an old Volkswagen beetle. There are a couple enemies but they usually run off quickly. If you jump down very quickly you may have to fight them. From where you initially dropped down you want to look for a blue wall across the street. There will be a gray garage door next to it and 2 carts in front of it. In between the 2 carts and in front of the blue wall there is an open manhole cover. Drop down into it and you can find the relic on the other side of the sewer right next to the gate.