The fire spirit the source of all heat in the world... Umm, I'm pretty sure that's an exaggeration right? Other wise it would have taken one slice and flames spewing while the monster was decapitated... OR it could be that the sword is legit but the hero or the person wielding it isn't using it to its full capability. Something like that.
@Velsy
Wrong. Largely because that sword is a cheap ass sword he got when he was 13 years old as a present from Reme for forming their party initially. He is contracted with the Fire Spirit and thus any sword he wields becomes the holy sword irregardless of how terrible it originally was, if stolen it would go back to being a cheap dull hunk of iron...presuming it didn't smite you for trying it. The only thing his holy sword could gain from is if he bought one made of something particularly sturdy. But because he is contracted to one of the 4 most powerful spirits in the world his sword has absurd attack stats, though theoretically since his is fire its probably even higher attack then the other four potential swords but they don't really compare them much in the series.