Hard Skin 
is an item that is required for most headgear quests at Justin.
Source: It can be obtained from Hardrock Mammoth (100.00%).
Market Price:
- 180 PC (As of Sep 16, 2018)
- 200-250 PC (May 17, 2018)
Monster Location: man_fild03 (Warper NPC > Fields > Manuk Fields > Manuk Field 3).
Monster Respawn Time: 4 hours after its death
Note: There is no way to know when Hardrock Mammoth will re-spawn unless you know its previous time of death.