User blog comment:JohnnyN64/Mysteries of Fairy Tail/@comment-117.195.57.105-20140612100501/@comment-24066070-20140612114147

I think God Slayers were just invented by the author for the sake of having something which surpasses Dragon Slayers, even if it only surpasses them in theory.

The author just needed certain mages whose element could not be easily consumed by the Dragon Slayers.

I don't really like the existence of God Slayer magic, since the existence of Gods hasn't been explained. Dragons exist/existed and Devils/Demons exist as well, each of them is clearly an independent race with its own characteristics that make said race different from humans. Therefore, it makes sense if there is a type of magic which specifically targets and exploits those differences.

However, just as others before me pointed out, there is no clear-cut definition of a God. It could be an uber-tier mage (i.e. Four Gods of Ishgal) or a sage whose only quality is a vast amount of knowledge. In either of these cases it would still concern humans, albeit skilled ones. This makes it difficult to say what God Slayer magic is.

In Feng Shen Ji, the Gods are a different race who posses a power (Transcendant Vigour) which humans don't posses. This clearly sets them apart from humans and the only known God Slaying power in the series is something which targets this Transcendant Vigour, making it extremely effective against Gods yet completely useless against anything that isn't a God.

In Highschool DxD, there are Holy weapons which are very effective against Devils and there are also the swords Ascalon and Gram which have Dragon Slayer properties.

These are just some examples of series where the difference between different races is established in a better way than in FT, where the existence or characteristics of Gods isn't even touched upon.