Here the poet has used two literary devices: simile and paradox. Simile appears in the second like where he has compared dead leaves to ghosts. Then you can see the use of paradox, as “ghosts” flee from enchanter is quite paradoxical. Normally we think that ghosts are terrible looking and therefore can cause fright and provoke flight. Nonetheless, here ghosts seem frightened themselves and fleeing from presence of enchanter. Besides, there is a phrase “unseen presence” which initially look paradoxical, yet it is only apparent because here the presence is in fact the presence of wind that can only be felt not viewed.
Simile and Paradox in Shelley’s “Ode to the West wind”