Nargott
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I listed the ways in the post, but in reality, the scales don't protect against deadly cold (the description says it protects against the demon's sword and the spirit's and archer's abilities). The demon's sword and the spirit's ability have been tested and confirmed to be effective. Cold has also been tested and confirmed to be ineffective. The charge attack shouldn't protect, and it doesn't, that's also been tested. As for the archer's shot, there's some confusion - according to the description, it should work, and when aiming the archer at the hellspawn, you can see increased damage, but I haven't tested it in practice; it just got scared instead of attacking.
Charge attack and archer's shot ignore defense; they shouldn't have anything to do with this ability.
There's an ability called "armor" that prevents defense from being lowered; that's what works against charge and archer attacks (but not against the enhanced arrow and the zombie's weakening aura).