I did this about 5 times with the same issues as everyone else, so by the sixth time I was fed up. So I started the quest, killed most of the beginning mobs (before the cave), and then thought 'screw it' and ran past every single mob following that. I did not kill a single thing and Tonn stayed with me and blew up all three towers. When it was time to clear the beach, I did so by killing everything on the beach (including all the krait) and it didn't complete, so I had to go up and kill the two abomination and the other mobs with them, and then it completed and WONDER OF WONDERS I could speak to Tonn! So the solution: Don't kill anything until you reach the beach. Good luck! (Oh, and I don't know if this is relevant, but I'm a minion necromancer and I didn't summon any of them for that successful run.)