Dying Light A Long Way Down Trophy / Achievement will give you one Bronze trophy or 25 gamerscore points. To get this Trophy / Achievement you must jump from the Infamy Bridge in the slums at night.
First of all its need to be night time for you to be able to complete the Dying Light A Long Way Down Trophy / Achievement. If it is not, just go to the safe zone and get into your bed, this will make the game start with night.
When it is night, reach the Infamy Bridge which is located at the east of the Slums, on the right of world map. Run to the end of this bridge and just jump off to unlock Dying Light A Long Way Down Trophy / Achievement.
If you are still not sure, watch the following video on how to get the Dying Light A Long Way Down Trophy / Achievement. This game is now available on PC, PS4 and Xbox One.