How does the narrator in "The Nun's Priest's Tale" make the story seem epic and heroic? How do the word choice and phrasing affect the narration?

The narrator accomplishes that by using a specific style of narration and a specific language. This story is an example of mock-heroic technique where trivial things are described as something unearthly and sublime. This is accomplished by using the epic language, that is the elevated language that is used to enhance the deeds of epic and mythic heroes.

The narrator in "The Nun’s Priest’s Tale" decides to tell a humorous fable about farm animals. He chooses the lofty style of epic narration to relate his commonplace story of a rooster and a fox. This style adds a comic element to the story. The nun’s priest compares the rooster Chanticleer to epic Greek heroes such as Hector and Achilles. Chanticleer’s boasts are like those of epic characters. Pertelote’s screams are likened to the loud shrieks of Hasdrubal’s wife on learning of his death in Carthage. The epic style is used to entertain readers and hold their interest in an otherwise mundane story.