formidability vs difficultness

formidability and difficultness both are nouns.

Word	Noun	Adjective	Verb	Adverb
formidability	Yes	No	No	No
difficultness	Yes	No	No	No

As nouns, difficultness is a hypernym of formidability; that is, difficultness is a word with a broader meaning than formidability:

formidability: impressive difficulty
difficultness: 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 formidability include difficulty.

formidability (noun)	difficultness (noun)
impressive difficulty	the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result