I couldn't help but overhear...There are Ogres at the 20th floor of the Demon Tower.The Drop Rate might be a lot less but at least you won't have to worry much about mana consumption and dying.By my estimations,you should be able to get 20 nails in 3 days if you hunt at least 25 per days.

'Tis such  a disappointment.
To think that Adventurers Academy's top student is someone so Demon Queen obsessed...
The Faculty's even expecting you to become the next hero who'll save the world!