"I shot down his arguments and buried them in the sand"

It is what is called a mixed metaphor, and as such, should be aborted and forgotten. It's bad writing.

I don't know that they are mixed. A mixed metaphor is the use in the same expression of two or more metaphors that are incongruous or illogical when combined, as in “The president will put the ship of state on its feet.”. I don't see why a shot-down argument can't be buried. I do agree it's bad writing.

Not every plane which is not down, nor for that matter, every individual who is shot down, in war time, suffers that fate in sandy deserts.

Metaphor/hyperbole.

If Setanta knew his ass from a hole in the ground he might be able to make such a judgment.