Okay, so this is what worked for me. After doing the quest multiple times with the methods mentioned above, someone in my Guild said that I should 1) clear all mobs going from 3, over the water to 2 and then 1, and then talk to Tonn, like one said in an earlier comment (also kill the 4 mobs in the water at tower 3). The difference is this: DON'T PUSH. Be just a little behind the NPCs and let them kill whatever spawns when they walk through. Take it slow. They are immortal in this quest so it's no problem.
