Now isn't that specific to nations with access to flying units?
And of course there are more ways, like remote summons, its just that its a lot easier when you get more specific. If we knew what nation he was fighting, we could suggest ways to defeat the PD since we'd know what it has, if he told us what nation he was using, it'd be easier to mention all the specific tactics you could use with them. Right now all we know is that there is a wall of PD that is apparently unbeatable. (Though of course it can't really be unbeatable.)