I've tried it 3 times in a row, the third time i finally managed it by killing the kraits last before talking to Tonn. The NPCs attack the kraits all the time so it was hard to pull them away. I didn't have a problem with blowing up the towers, though first time a risen was stuck inside the first blown up tower, and was hard to kill that way. Second time I managed to pull him out before the tower exploded.Conclusion: possibly always following the marmox and keeping the party together may be the key to successfully explode the towers, killing kraits last does seem to be the solution for the Tonn dialogue. Still, it's annoying. If they need to be killed last for whatever reason, let them spawn last.
