What language technique is 'She is pure guts and steel'?

It's a sentence, not a phrase, but I don't think that is the answer your teacher is looking for.

"Guts" -- which literally means "intestines" but is used to mean courage -- is an idiom, as you suspected, or more exactly a figure of speech.

"Pure guts" is hyperbole, an exaggeration for rhetorical effect.

"Steel" is a metaphor ("the use of a word or phrase to refer to something that it is not, invoking a direct similarity between the word or phrase used and the thing described").

Perhaps the technique referred to is one of these.