Why would Ainz use Body of Effulgent Beryl (which According to the Wiki reduces the effectiveness of bludgeoning attacks) as his defensive spell when fighting Shalltear as opposed to a spell that reduces piercing damage? Such as Body of Effulgent Aquamarine. Given the weapon she is using, it seems it would deal piercing damage as opposed to bludgeoning damage.
This seems especially true in this screenshot where her pipette lance clearly pierces him.