Okay, so I'm trying to figure out whether there's an exception to the rules, or a specific order of operations to the math to make a babau's protective slime a threat to any manufactured weapon. Here's the ability's text:
Protective Slime (Su)
A layer of acidic slime coats a babau’s skin. Any creature that strikes a babau with a natural attack or unarmed strike takes 1d8 points of acid damage from this slime if it fails a DC 18 Reflex save. A creature that strikes a babau with a melee weapon must make a DC 18 Reflex save or the weapon takes 1d8 points of acid damage; if this damage penetrates the weapon’s hardness, the weapon gains the broken condition. Ammunition that strikes a babau is automatically destroyed after it inflicts its damage.
Okay, so it does damage to natural attackers/unarmed strikers if they fail a Reflex save based on the first sentence, and automatically destroys ammunition based on the third sentence. Fair enough.
The second sentence is where there seems to be an issue of futility and useless dice rolling. At most, if the wielder of a manufactured melee weapon fails the Reflex save, their weapon has to contend with 8 points of acid damage, and if that penetrates the weapon's hardness, it gains the broken condition. Just looking at the table for damaging objects, no blade or metal-hafted object or heavy shield has anything to worry about, as they all have a hardness of 10. The remainder (non-metal-hafted weapons, light shields) have a starting hardness of 5 (they gain +2 hardness for every +1 enhancement on the item, so they frequently won't be subject to this either).
But even for that pool of manufactured weapons with a hardness of 5, we also have to consider the following rule:
Energy attacks deal half damage to most objects. Divide the damage by 2 before applying the object’s hardness. Some energy types might be particularly effective against certain objects, subject to GM discretion. For example, fire might do full damage against parchment, cloth, and other objects that burn easily. Sonic might do full damage against glass and crystal objects.
So, on that basis, since the 50% calculation is applied before the hardness is factored in (explicitly stated as being the correct mathematical order of operations here), the babau's protective slime is doing no more than 4 (1d8/2) acid damage to a weapon. Does this mean the text in the second sentence of the protective slime ability is just wasted words, and no one wielding a manufactured weapon has anything to worry about? Or was the intent to ignore the halving rule for this kind of ability so that it has at least a puncher's chance of doing something?