Blunt head trauma is almost always depicted as a way to conveniently knock a person out in television and books instead of the actual death sentence they are in real life. That's just how it is. Alternatively, it might've been a giant rubber mallet, because even I agree that using a sledgehammer is way too overkill and there's no way the author didn't know that.no one gonna question how a straight up SLEDGEHAMMER DIRECT HIT on the back of his head didnt kill him? Or by Miracle just give him some brain damage? its a fuckin Sledgehammer. its way heavier than some iron pipe or baseball bat. for plot convenience only the bloody Tattoo is the only scar Maru got.