Draco, you are 100% correct on all accounts... except this one. A sword-board fighter or Champion can raise a shield to get a AC20.
Fair point. I also haven't done the math on monks in a while.
By shield-blocking once each-round. This will tank some of the damage.
Also true, but I was going with a pretty simplistic scenario. I'd be happy to see the details (including raw rolls) of one of your simulations.
I'll note that shield block math is very very advantageous at low level and that falls off strongly past 13th or so (even minimum damage hits will bypass hardness at least a little bit, whereas at 1, a shield can tank even a max damage hit).