If this broad description sounded intriguing to you, I encourage you to read a sample or pick up the light novel yourself.
I have read the first 5 volumes. The first two volumes, also known as the Gaeabrande Arc, introduce us to the our main two characters and we learn a great deal about their personalities. The first volume is filled with wonderful comedic moments while the second volume takes a sharp tonal shift near the end. With the way that it's written, there is a good deal of foreshadowing and the "twist" doesn't feel awkward. Elulu and Mash are also wonderful supporting characters and the story feels rather lighthearted.
For the Gaeabrande Arc, I would give an 8/10 for my deep enjoyment and satisfying emotional investment.
Volumes 3-5, the Ixphoria Arc, deviate greatly from the light and occasionally serious mood of the prior arc. The world described by the authors in this arc is extremely dark and would definitely be rated R if adapted into anime. There are the usual comedic moments between Seiya and Rista in these volumes, but they often feel uncomfortable given the circumstances they are put in. I also believe that the supporting cast is much weaker, consisting of strange yet unintriguing characters.
Volume 3 has this bizarre revelation at the beginning, only to be nullified, and then restored 'halfway'. The reasoning and outcome of these events feel inexplicably confusing but did not heavily impact my enjoyment of the novel. The grim setting of Ixphoria is really pushed by how Seiya also treats other humans worse (knocking old women unconscious) than he did in the first arc (threatening shopkeepers). I will commend several emotional scenes for not feeling forced or out of place.
The reason the darker theme for Ixphoria arc overall doesn't work well is because the struggles feel less meaningful. In the Gaeabrande arc, Seiya would rarely struggle because, well he's 'overpowered'. As a result, whenever he struggled there was a lot of tension. In the Ixphoria arc he is ALWAYS 'struggling', and it doesn't invoke the same emotional tension.
In volume 4 especially, all the tension is obviously fake. The story in this volume particularly feels like a playthrough of Fire Emblem where the characters barely move and they are just turtle-ing to finish every level. This is very much an in-character strategy for Seiya, but there is much less enjoyment to be had.
edit: the following portion of my review has been rewritten after reading volume 5
The structure of volume 5 is very reminiscent of volume 2 with how Seiya takes a very active role in saving the world and contains an emotional plot twist near the end. Unfortunately the foreshadowing for this twist is lackluster and left me dissatisfied upon reading it. However, I do like how the author used the plot twist to give greater emotional tension to the situation (I like how it was used, not how it was set up). Volume 5 also concludes the Ixphoria arc on a satisfying note and setups the next one rather well.
For volumes 3, 4, 5 I would give a 6.8/10 largely due to the theming issue I mentioned previously but the fifth volume redeeming the arc.
Another thing I would like to discuss about this light novel is HOW it is written. The author very effectively keeps the characters moving between locations at a good pace. There isn't a massive amount of time used to describe intricate details and there is never too much dialogue on one page. For example, you are never told what "Atomic Split Slash" or "Maximum Inferno" do, but you can easily assume just based off the names what kind of skill they are. You also aren't bogged down with stats like attack, defense, hp, etc. particularly in later volumes.
Essentially, this light novel is a pleasurable read and prioritizes progression over information appropriately.
All in all, I would give this Light Novel a 7/10 for its wonderful first arc but a somewhat bumpy second arc.
