The Hylian Shield is the best shield in the game with 90 defense and the highest possible durability. It’s located inside of a treasure chest somewhere in Hyrule’s Castle, more specifically the ‘Lockup’ section on the west side of the castle. It’s a dungeon area, so don’t go searching too high.
Ganon does not need to be faced in order to obtain the Hylian Shield, but the treasure chest is guarded by a tough beast called a Stalnox. The moment Link enters the small room, the gate will lock behind him and the Stalnox will appear. Aim for the eye, as it is the Stalnox’s main source of power. After defeating the monster, open the treasure chest to receive the shield.