To reach this location, you can start from the Hyrule Garrison ruins and take the path leading southeast. When you come to a crossroads, take the path to the right that stretches towards the south ...
The second piece of Phantom Armor, Greaves, is hidden in an EX-treasure chest buried in Hyrule Garrison Ruins. This area is to the South of Sacred Ground Ruins and is a graveyard of Ancient Guardians.