Trapped: MacBook always shuts down during EFI SMC Update 1.8. Can't boot. How to remove updater from boot routine?
EFI firmware updates are 'staged' in the normally hidden EFI partition on your disk, which typically resides at device /dev/disk0s1.

Once in single-user mode, you can create a folder in which to mount this partition, then mount it like so:

mkdir /Volumes/efi
mount -t msdos /dev/disk0s1 /Volumes/efi


From here, navigate into the /EFI/APPLE/FIRMWARE/ sub-folder of the EFI partition.

If you find some files here, probably these are what your MacBook is trying to process and getting stuck on. You can safely delete them and reboot.