Same happens for me. Killed him 2 times now and the quest wont complete.
Gonna try a complete game shutdown (and not just a “leave game”) to see if that “fixes” it.
EDIT: Nope quit game did nothing to fix this
EDITEDIT: Found a post of the US forum, that suggests killing Astaroth near the western gate in the last area.
However odd that sounds - it worked for me after 2 more kills with no quest progression.
For whatever reason it seems that you need to be “close” to Donan for the kill for the quest to progress.
Just so odd that things like this cant be avoided and even more so, frustrating that it happens during pre-launch. Stuck now I guess till they fix the fight.
Unbelievably annoying!