I fixed it by killing everything but the Kraits before talking to Tonn, then (important) do not get close enough to the third tower to activate Tonn blowing up the tower, kill the 2 waves of kraits and risen first. There may be some Kraits in the tower as well so you may need to inch around the tower and tag them somehow without activating Tonn. Then get in range of the tower and some more risen will spawn after it blows up. This worked for me.