So after the struggle with the summons I started pounding the dragon with Boulder Bash. I F5'ed before casting it and F8'ed if he wasn't knocked down (none of my characters had Drain Willpower so I had to resort to that).
Drain Willpower wouldn't have helped you there anyway, knockdown is governed by the Body Building saving throw. Disease or Soul Sap would have been the go-to debuff there.
