In our saga, we just use: 0 as a defense total (and no roll) when the target is defenseless and in combat. (Such as a surprise arrow or whatever)
And when someone has multiple wounds for more than -10 malus, we usually make that character fall unconscious.
For unconscious characters, we just consider their soak vs atk+dmg, and humans are like easily killed (slit throats everywhere) but dragons and such creatures? not so much. It is still feasable, but a lot harder than killing a human.