I see two other ways to make this easy, both of which have worked for me in the past :

1) use grub rescue cd (which you can put on a USB stick). Then fairly easy to sort out by picking the right menu options.

2) use these incantations, lifted from a post elsewhere :

mkdir /sysroot
mount /dev/your-root-dev /sysroot
mount /dev/your-boot-dev /sysroot/boot
mount --bind /dev /sysroot/dev
mount --bind /sys /sysroot/sys
mount --bind /proc /sysroot/proc
mount --bind /run /sysroot/run (recommended if you are using systemd)
chroot /sysroot
grub-install /dev/your-grub-boot-device (may be grub2-install on some distro)