I also conform that Opus Holder s method works. When I cleared all enemies but missed 1 enemy near the1st tower, it was trapped when Tonn destroyed it. The quest couldnt be completed. The point is to clear all enemies near the towers before the quest starts. There are some in (!) and behind the towers and in the water, who can be trapped when the towers are demolished. I casted AoE on the towers to see if anything remains. Oh i also revived the sylvani and talked with her before the quest begins..
