subtlety vs difficulty

subtlety and difficulty both are nouns.

Word	Noun	Adjective	Verb	Adverb
subtlety	Yes	No	No	No
difficulty	Yes	No	No	No

As nouns, difficulty is a hypernym of subtlety; that is, difficulty is a word with a broader meaning than subtlety:

subtlety: the quality of being difficult to detect or analyze
difficulty: the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result

Other hypernyms of subtlety include difficultness.

subtlety (noun)	difficulty (noun)
the quality of being difficult to detect or analyze	an effort that is inconvenient
a subtle difference in meaning or opinion or attitude	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result