Enlightenment

In full Colin Plate Armor w/ Vales Shield and r1 Heavy Armor and Shield Mastery, I've reduced a Giant Lion's Stampede attack down to 1 damage. That give some sort of idea of how effective it is?

That's because the Vales Shield reduces everything to 1 with Defense loaded, even if you have Rank F Defense and 0 Protection. The reason why it's like this is unknown but it's being speculated as an unintended glitch on the developers part, especially the fact that a Rank F Defense with Vales Shield renders a Master Lich's attacks to 1. One thing to note though is that it doesn't work in duels. On the other hand, a Medium Shield such as a Kite Shield or a Tikka Shield will reduce damage with Defense loaded as well. Although it's not reduced to 1 damage, it's still a reduction.

I'll try it again with Shield of Avon and Valkyrie Shield then next time I see it.