Still bugged, but the fix is pretty simple. Some others have mentioned it before me. I managed to complete it on my second try. On the second tower you just need to make sure all the enemies around the tower are killed before Tonn places the bomb so none of them get stuck beneath it and cause him to get stuck waiting for them to be killed. Range attacks are your best bet for this. There are a couple around the back of the tower you'll need to draw out. Do the same at the third tower - kill all the krait etc, and you should be good to go.
