As far as I know, 0 Defense just means your opponent will hit every time. The Attack vs. Defense check comes first and then the (Weapon Damage + Strength) vs. Protection determines the base damage. If you have 0 Defense, though, I believe that may mean you're getting stung by the poison with every blow made against you.
So then, does poisoning require the attacker to penetrate armor? Seems like it should, but it might not.
Not that it helps any, but I was just playing a Wyrm-led Rl'yeh and had no problems with him chomping Titons on the front row. I don't recall him ever getting poisoned or having any real problems at all. He just marched about, swallowed them up, and finally regurgated the lot onto the Ulmish shoreline. (Well, not the Last, actually, but that's what I was imagining! Heh!
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