Think I nearly got one shot by a 2-star Draugr with a bow? While blocking (I thought with a bronze shield?). Unless it was right on the edge of me bringing my shield upyou lose 5%, and that's quite a lot in the higher skill areas.
But what mobs can oneshot you through what shield? Only Fagling berserkers come to mind, if at all.
Yeah, furgling with a spear as well, I think.