If what you need to do is take a screenshot of the running app with the ad loaded and displayed within it, that is something that can be achieved.
I would recommend setting up a new Ad Unit ID which will be used solely by a developer for this purpose. Add a single line item for it within DFP that points to the ad you'd like to capture. Then the developer can load the source code for the app from which you are taking screenshots, swap the new Ad Unit ID in for another, and run the application. You would then see the ad in the placement for which you swapped ID values, and can take a screen capture.
This would, in effect, be targeting a single device, since you would only use the Ad Unit ID with one device (or the simulator).
-Andrew