fight off vs repulse

fight off is not a noun while repulse is a noun.

Word	Noun	Adjective	Verb	Adverb
fight off	No	No	Yes	No
repulse	Yes	No	Yes	No

As verbs, fight off and repulse are synonyms defined as:

Other synonyms of fight off include drive back, rebuff, repel.

fight off (noun)	repulse (noun)
	an instance of driving away or warding off

fight off (verb)	repulse (verb)
force or drive back	force or drive back
	cause to move back by force or influence
	be repellent to; cause aversion in