Okay, maybe Bickslow wasn't a good example but how about Laxus vs Alexei/Ivan? If we're talking about overpower, I can ask the same question. Why was Laxus being affected by paper?
Laxus being sliced by a piece of paper is no different than Mirajane being bruised by books. I mean really, Alexei/Ivan is way weaker than Seilah. Also, most people would say that Laxus is stronger than Mirajane. So my question here is, why couldn't Laxus just casually stand there and catch all the paper with his bare hands? Or why didn't he just let it hit him instead of defending against it since no damage will be done? How about just burning them up? You see, objects associated with magical power are quite different.
Similarly, if you're arguing why Mirajane cannot just destroy the books, then why can't Natsu just evaporate Juvia's water?
Even so, water shouldn't cause damage to anyone except when they drown in it. So the fact that Juvia's
Water Jigsaw can still cause some damage without having to drown her opponent, means that objects powered by magic are different. The damage of the attack just depends on the user's MP.
Water Slicer
Here it shows Gray being hit/cut by it. In the anime, it sliced a stone column.
Water Nebula
Water Claw
So why is the water not that strong? Because Juvia's magic power isn't that high. So how can Seilah's books do damage? Because she has high attack power.
You're right here. I seemed to have overlooked that. I guess what I meant was that the books were fueled by Seilah's curse power (strength) which is why the speed at which it was thrown was faster than if Lamy would have thrown it, thus it does more damage. I guess this situation is more comparable to how a steel ball thrown at incredible speed would go through Silver.