Delete large numbers of photos at once without deleting from Google+ backup

I have about 5000 photos on my phone right now, and they are all backed up on Google+. I know I can delete photos without removing them from Google+ by using the Photos app and selecting "On Device." I also know that I can select multiple photos by selecting one, and then clicking more. However, to quickly remove all photos, say, older than a month, I would need to individually click on 4000 thumbnails. Plus, when I do this for just 10 images it takes ~10 seconds to delete them. This would be a nightmare to do for 4000 photos. Is there a way to batch-delete lots of photos from my phone, while ensuring that they won't get deleted from Google+? (Alternatively, is there a way to delete-all locally, and selectively re-add photos from Google+?) --- Edit (2017): This is obsolete ever since Google added the ability to "Free up space" from the menu in the Photos app.