...with the pressure washer. You can try a lower PSI to see if that will help with maybe a different pattern nozzle.
If you're repainting the deck, you don't need to remove all of the paint. Just rough it up. If you're thinking that you're going to remove all of the paint in order to try to stain it, then you absolutely have your work cut out for you. And it won't be easy, if it's possible at all.