You can know the hardest by knowing some latin (Telos means Endless).
By knowing some mythology, you can know almost all difficulties:
Orpheus, you know his story ( he descended in hell to recover his lover Eurydice )
Orthrus, the mythical child of Nyx, brother to Cerberus, Sphinx and Chimera, that was slain by Hercules ( so, after a "normal" human, there is a mythical beast)
Thanatos, the God of the Peacefull Death and Eternal Sleep (another child of Nyx) (pretty logical)
Asura is taken from hinduist, buddhist, vedic and zoroastrianic. Referring to Vedic, King of asuras, and Zoroastrian, mispelling of Ahura Mazda, much like YHVH for Cristians. (Remember Soul Eater Asura too)
Orpheus Telos - as Telos means Endless, and Orpheus received a gold lyre from Apollo
Another way:
The Star Rating order: I made it all to preserve the order of the difficulties.
This doesn't deserve a rule. Neither a Guideline maybe.