difficulty vs exertion

difficulty and exertion both are nouns.

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

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

Other hypernyms of difficulty include effort, elbow grease, sweat, travail.

difficulty (noun)	exertion (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