2 of them did not take any damage ? That's odd, but if they were indestructible, you wouldnt have destroyed one of them. Maybe you are not supposed to destroy those, not sure about that part of the story anymore. Did they have specific names? Anyway, AI cant be changed.
Portalearth Not like forlon and other threads didnt have downloadable saves at last vanilla mission for people with issues doing them for being "too hard" or something.