struggle vs fight back

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

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

As verbs, fight back is a hyponym of struggle; that is, fight back is a word with a more specific, narrower meaning than struggle:

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

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