Apple - How to delete all photos in the camera roll in iOS 8.x?

If you have a Mac, connect your iPhone to it.

There is a build in app (from Apple) called Image Capture in your Applications folder.

Use it to bulk delete Photos on iPhone.

You can use Image Capture... However, if you see a cloud next to the name of your device then you need to disable iCloud Photo Library before the delete all button will appear.

iPhone: Settings -> Photos & Camera -> iCloud Photo Library off Settings -> Photos & Camera -> My Photo Stream off

Then, try Image Capture again and the delete all button will appear.