Pretty much impossible.
I think Obama had to lose a few counties in 2012 with less than token opposition.
Obama lost counties in 2012. The odds of this happening anytime are zero.
I don't say that Hillary will win every county, but the comparison between Obama '12 and Hillary '16 is a bit lame.
Obama is thoroughly despised by fellow Democrats (!) in the Greater Appalachia area, where he lost numerous counties.
Hillary doesn't arouse
such any hatred among Democrats. Her only haters dwell on the Internet and, in particular, well, here on Atlas...