The problem with me seems to be the same problem that was there months ago… there is a Risen Thrall that gets trapped under the second tower when it is demolished, so you cannot complete. You cannot focus on the npc when the tower is on top of him and attempting to kill it does not work, so you can never finish.
My work-around back then is the same one I just attempted and it still works… just go kill all the Risen NPC’s BEFORE talking to Tonn and starting the mission. Just work your way up the beach, kill them all and go back and talk to Tonn. Tedious, but it works and if you are careful isn’t too bad solo.