No way without downtime. You need to enter the BIOS during reboot, using the serial connection. Press any key when prompted, then select to format the disk.
Afterwards, the firmware image, signature databases and configuration will be gone!!
So prepare this operation and have the firmware image and config file ready.
The most convenient way to reload these, given a non-functional FGT without IP address, is to copy them to a USB stick and reboot the FGT with the stick attached. The boot process will compare firmware version and config and restore one or both if necessary.
The files on the stick need to have these filenames:
for the config: "fgt_system.conf"
for the firmware: "image.out"
Other files will be ignored.
"Kernel panic: Aiee, killing interrupt handler!"