The thing is, when the maximum difference between your hero's defense and the enemy's attack is reached (which can be seen from the formula you provided – a coefficient of 0.3), no spell that modifies defense – including the Expert Shield – will work. The total damage will still not drop below 0.3.

Interesting, I didn't know about this nuance. Maybe I have a very old version of the physics engine, but I didn't notice this fact there (if it's not actually in the manual, although you know better, then maybe it should be added. Moreover, in the version of the physics engine that I have on my computer, an example is considered with a cap of 0.3 and the Expert Shield, and the multiplier of 0.7 is present). And in the real game, the chance to notice it is extremely rare. But even just 21% damage at level 20 seems like a sufficient boost.