Well, no luck with fastboot so far due to droidboot limitations and some problems with tools.

To make it easier, I did an improvised installer, based on Archiso and custom script. Just download the archive, extract it to the root of FAT32-formatted USB drive, and boot tablet from it:
https://mega.nz/#!8FZRiBwB!FBBD8CUMa...Qp713VqT-FoAhM

One thing though, the tablet of yours is ONDA V820W V5, while mine is V3. There are slight hardware changes, but I'm not sure what is different exactly. The image should boot, but touchscreen/Wi-Fi/Bluetooth/sound/etc may not work depending on what has been changed.