While not strictly a "Renaissance novel" (as the Renaissance period is typically considered to be from the 14th to the 17th century, and the novel was published at the end of this period), it is one of the most influential and important novels of all time and is often considered a defining work of the Renaissance.
Don Quixote is a story about a man who, obsessed with chivalric romances, believes himself to be a knight errant and sets out to right the wrongs of the world. He fights windmills, believing them to be giants, and sees inns as castles.
Here's why it fits your description:
* Renaissance setting: While the novel was published at the end of the Renaissance, its themes and the character of Don Quixote are deeply rooted in the Renaissance spirit of humanism, individual agency, and the pursuit of ideals.
* Man fighting imaginary enemies: Don Quixote's battles are entirely imaginary, fueled by his distorted perception of reality based on his romanticized view of chivalry.
Let me know if you have any other questions!