If you're really borked you guys can try the procedure here:
This is for when DFU doesn't work. The procedure erases, writes the Soft Device, bootloader and then the system part and Tinker app. It doesn't matter how borked your device is (firmware wise). You should always be able to get it back running shape.
P.S. this works for all Particle Mesh based devices with the NRF52840.