Delete large number of pictures on iphone

From the Recents album, lightly tap the last photo and drag your finger upward to capture multiple images at a time. you can then delete all of the selected ones at once using delete. You'll still have to go into recently deleted, but there is a delete all command there.

You could also try using image capture app from launchpad and see if you might be able to delete all of your images using image capture. You may be prompted to download to your Mac, so put them into a temporary folder somewhere (desktop, etc.) that you will delete after import. One option should be to delete after import.