Run script at shutdown/reboot with systemd
Just copy / link the script into your /etc/init.d/rc0.d (HALT) and /etc/init.d/rc6.d (reboot) directories, and make sure it's name is prefixed with: "K20"
FYI: http://www.linux.com/news/enterprise/systems-management/8116-an-introduction-to-services-runlevels-and-rcd-scripts