Are you using iCloud Photo Library? If so then don't delete the photos from your iPhone - they'll be deleted from iCloud and any other device linked to your account. If you select "Optimize iPhone Storage" then the capacity taken up by your photos will reduce.
If you're not using iCloud Photo Library, and are still using "Camera Roll" as a local storage only, then deleting photos from this album has to be done one at a time unfortunately (hence the recommendation to disposition your photos frequently and not leave them in Camera Roll). Other than deleting one at a time, you could restore your iPhone as new and not load your backup ... as harsh as that sounds, it may be a faster option for you. Of course you'll have to re-do all your settings and lose some data like recent messages, call history, app data, etc.