Kingdom Rush Vengeance (Steam) Impossible Hero Challenge - Kazan Mines Iron

I keep checking my settings to make sure I'm actually playing on Impossible difficulty.  So far Vengeance is far easier than Origins, as by now in Origins we had had some moderate challenges that at least distinguished our heroes a little bit.  As it is we put another round of trivial 'A's in the bag and hope for more on the next map.

Kingdom Rush Vengeance Impossible Hero Challenge - Kazan Mines Iron