fight back vs struggle

fight back is not a noun while struggle is a noun.

Word	Noun	Adjective	Verb	Adverb
fight back	No	No	Yes	No
struggle	Yes	No	Yes	No

As verbs, struggle is a hypernym of fight back; that is, struggle is a word with a broader meaning than fight back:

Other hypernyms of fight back include contend, fight.

As verbs, struggle is a hypernym of fight back; that is, struggle is a word with a broader meaning than fight back:

Other hypernyms of fight back include contend, fight.

fight back (noun)	struggle (noun)
	strenuous effort
	an energetic attempt to achieve something
	an open clash between two opposing groups (or individuals)

fight back (verb)	struggle (verb)
fight against or resist strongly	be engaged in a fight; carry on a fight
defend oneself	to exert strenuous effort against opposition
	climb awkwardly, as if by scrambling
	make a strenuous or labored effort