Tips for killing the Mother Plant in Strangerville?

I have dispatched the Mother Plant a couple of times before but  in my current game I am having quite a different exerperience.   Does the Luck factor have any influence now in whether you have a better or worse chance of killing the monster now?  I rather doubt they went back and reprogrammed an old pack to be influenced by the newer packs but it does seem to be behaving differently.  Me and my team often get the plant almost dead and then she miraculously revives and suddenly her status is perfectly healthy again.  Is this a glitch?  How much vaccine should I have before starting my attacks?  I usually have 6 bottles but maybe I should just make it more like 10 or 20 because the team seems to run out of vaccine and then they run for the hills.