surprise attack vs onslaught

surprise attack and onslaught both are nouns.

Word	Noun	Adjective	Verb	Adverb
surprise attack	Yes	No	No	No
onslaught	Yes	No	No	No

As nouns, onslaught is a hypernym of surprise attack; that is, onslaught is a word with a broader meaning than surprise attack:

Other hypernyms of surprise attack include attack, onrush, onset.

surprise attack (noun)	onslaught (noun)
an attack without warning	(military) an offensive against an enemy (using weapons)
	the rapid and continuous delivery of linguistic communication (spoken or written)
	a sudden and severe onset of trouble