Delete large numbers of photos at once without deleting from Google+ backup
You should be able to plug your phone into your computer and browse your files (probably under a folder called DCIM) using Windows Explorer (Not Internet Explorer but the file explorer) or Apple Finder.  You might need to install the specific drivers for your phone if it doesn't automatically connect.  

You can then sort your pictures based on date and remove everything past a certain date.  They will not be deleted from Google+ since you are doing a local delete.

You can then download photos from Google+ that you want to keep locally.