You completely misunderstood what Arlong was saying. Luffy is practically immune to blunt force attacks unless they are imbued with Haki because he is made of rubber. He is susceptible to cutting attacks though because you can cut rubber, ergo that was his "weakness" pre-timeskip. It was not meant to be taken in the same vein as let's say, water being fires weakness.
Luffy is actually much more resilient to cutting attacks than normal humans, because rubber is denser than human flesh. Thats why when Kaku and Kalifa hit him with a double rankyaku, at point blank range, that completely sliced the brick wall behind him in half, it did minimal damage to Luffy himself.
Nice try though. You get an A for effort. :cheers:
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That holds true when he is slashing with his swords normally. But he can't control the cutting power of the Tornado after he has made the Tornado and is is going on it's own. He created a Tornado that had enough force behind it to cut through steel, yet the Fishman were left intact.
This is a Shonen manga. Characters are much more resilient than people are in the real World.