What are the literary devices in I Am Number Four?

Simile: "The power deflects into the trees which dance for a brief moment like the grains in a wheat field."
A simile is a literary device in which two dissimilar things are compared using the words "as" or "like." In the example from I Am Number Four cited above trees are likened to dancing grains in a wheat field. This is a particularly effective simile as it emphasizes the sheer force of the power as it crashes into the trees, and also the relative fragility of those trees in relation to that power, the kind we might associate with swaying grains of wheat.
Metaphor: "[I]n the waning light I see a river curling gently around that seems to cup the city."
A metaphor can be defined as a figure of speech in which a word or phrase is applied to an object or action to which it is not literally applicable. In the above example the river appears to cup the city. It cannot do so literally, of course, but metaphorically speaking the use of the river in this expression makes it seem that the city is protected and secure.