difficulty vs elbow grease

difficulty and elbow grease both are nouns.

Word	Noun	Adjective	Verb	Adverb
difficulty	Yes	No	No	No
elbow grease	Yes	No	No	No

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

Other hypernyms of difficulty include effort, exertion, sweat, travail.

difficulty (noun)	elbow grease (noun)
an effort that is inconvenient	use of physical or mental energy; hard work
the quality of being almost beyond one's ability to deal with and requiring a great effort to achieve a positive result