New Mac app Phone Exapander might have the solution you need. I just saved 1.22GB from my 64GB iPhone 6 and I didn't touch a single app, photo, video, or music file – all I used was the clear temporary files tool.
The clear temporary files tool allows you to clear the cache of apps on an app-by-app basis and you'll be amazed just how much stuff some apps are storing on your device's hard drive. The remove apps tool does exactly what it says on the tin but will handily list the largest apps at the top for you. The clear photos tool looks pretty nifty, it backs up photos — and/or videos — to your Mac taken prior to a date of your choosing before deleting them from your iOS device.
It's a shame that there is a need for an app like this; but until things change with storage management on iOS, PhoneExpander is going to be very useful to have around.