Need to reinstall GRUB2, but it requires me to boot into unbootable system!

This is not going to be straightforward so bear with me.

I cannot give you specific instructions as each system is a bit different but here are the major steps you need to do to fix your issue.

• The first thing you need to do is boot the system from a live medium. Any Linux distro should work as long as it has the same architecture of the installed distro.

• The second thing you need to do is chroot to your already installed system. I will lay down this example and this one for you to have an idea.

You will have to mount all the necessary directories first before you are able to chroot.

Once you are chrooted in, you may think of the environment as your installed OS. You can make the changes necessary, install grub both on the OS and on the boot sectors and configure it as needed.

Once you have done these steps successfully, you will have a fully working and configured grub.

You can probably fix this problem by reinstalling grub. You need to mount your existing root directory to /mnt and then arch-chroot /mnt. Then you can follow the instructions at GRUB- Arch Linux Wiki to reinstall grub. If you absolutely cannot get grub to work, you can always try a different bootloader.