The most obvious literary device used in lines 2-3 of Percy Bysshe Shelley's "Ode to the West Wind" is asimile-- that is, a comparison using the words "like" or "as." The third word of line 3 clearly signals the presence of a simile:
Thou, from whose unseen presence the leaves dead
Are driven, like ghosts from an enchanter fleeing . . .
One might also argue, however, that the idea of "ghosts" fleeing from an enchanter is also paradoxical. We normally think of ghosts as causing fright and provoking flight. Here, however, the ghosts themselves seem frightened. Further evidence of paradox may be found in the idea that the ghosts are fleeing an "enchanter." The word "enchanter" often has highly positive connotations. The enchanter here, however, is not only frightening but is sofrightening that even ghosts flee from its presence.
The phrase "unseen presence" might also, initially, seem paradoxical (how can something that is present remain unseen?). Yet this paradox is only apparent, since the presence being described here is the presence of the wind, which can be felt but not viewed.