Mass produce is a bit of a leap. It could just as easily be a rare relic he looted of the corpse of a dead nun and he only has the one. Also it could very much be presumed as well that these items only work as well as regular swords for people that don't know how to use purification techniques. If it could be mass produced easily, than people would take up arms to at least attempt to defend themselves instead of having just 2 sisters be sent to fight off the monsters.
The rest of the things I agree with and also said before. While physical force should work on the monsters, it is more than likely that they are way stronger than normal people and sending regular warriors at them would be waaaaaaay less effective than sending the sisters.
This might be a bit of over thinking, but the boulder trap only does like 20 damage to Lilia while it kills the goblin in a single hit, and Lilia can also tank a number of hits from deadly weapons like spears and meet cleaver and the like. Honestly a boulder swing like that could easily cave in full plate armor. So this sort of feels like extra evidence that their purification technique serves even as level of physical protection way beyond what even a fully armed knight could muster, which puts into perspective how physically powerful the monsters might be that do similar or even higher damage numbers. (I sound like a power scaler now XD)