If pacman is gone somehow, like if you run % pacman -Qs pacman it gives you an error,
Boot the obarun iso, mount the root partition of your system into /mnt (ie let's say it is /dev/sda23 blkid will help you identify which partition the system is in)
sudo mount /dev/sda23 /mnt
sudo pacman -Sy pacman -r /mnt
Your pacman will be back
edit /mnt/etc/pacman.d/mirrorlist and select your neighborhood's mirror.
To test before rebooting from the live session:
% sudo arch-chroot /mnt
% pacman -Syu