First: To reset the device first do the backup what ever you need.

https://www.linkedin.com/pulse/backup-restore-your-android-phone-using-adb-ajibola-okubanjo/

Second: this could make your phone able to flashboot and reflash  again:

You can send intent MASTER_CLEAR in adb:

adb shell am broadcast -a android.intent.action.MASTER_CLEAR

or as root

adb shell  "su -c 'am broadcast -a android.intent.action.MASTER_CLEAR'"

https://stackoverflow.com/questions/14685721/how-can-i-do-factory-reset-using-adb-in-android 

C:\Flashtool\x10flasher_lib>adb shell

shell@E2303:/ $ am broadcast -a android.intent.action.MASTER_CLEAR
am broadcast -a android.intent.action.MASTER_CLEAR
Broadcasting: Intent { act=android.intent.action.MASTER_CLEAR }
Broadcast completed: result=0
shell@E2303:/ $