Finally managed to get through this, 5th time lucky I guess. I had tried running through clearing everything (bar the krait) but didn't have any luck. What helped on my last attempt was the CTRL key which highlight all enemies in the vicinity, turns out I kept missing a risen on a little island between the 1st and 2nd towers (there are also a couple that hide in bushes and are easy to miss). I also made sure that after blowing the last tower I killed the regular risen that spawn as fast as possible before killing the krait last. I was also checking that I could "talk" to Tonn periodically whilst escorting them. if you find that he has changed to "greet" then you may have to reset.
