I love the core gameplay here! Very cool idea.
I think the balance could be improved though - if there are a couple of tanks that can see you and there are no buildings nearby, there's not much you can do to save yourself. Maybe the kaiju could have more health, or maybe tanks could take a turn reloading between shots? Or maybe their accuracy could decrease more at long distances.
I think it would have a bit more replay value if it kept track of your best score of buildings destroyed/civilian deaths too :) but overall really like it!!