Overall thoughts -
Boushokuki no Ken, overall feels like a missed opportunity that could be a lot better than what it is. A Fantasy title marred by mediocre progression of the story. It truly is mediocre, because as the reader continues to read it, it's all the same old formula in many other shounens.....and that's the problem. It claims to be a Seinen, yet the story overall feels like a typical Shonen manga. Well I guess adding in some adult themes of blood and big tits probably makes it a Seinen, which to be fair, shonens add plenty of fanservice too no doubt. But Seinen mangas have to typically rely on adult theme stories more than fanservice. I feel like this could have hit a better potential if was a Shonen. Or maybe if the story had a lot more themes that would be an interesting read for young adults. This seems like it targets neither, not shonens or seinens. It feels like it targets both, but misses both.
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On the art -
The art is fantastic, the mangaka has poured in their skills as an artist quite well. The amazing skills of the artist is on full display in the storyboards of the action panels. The mangaka has a good grip on perspective drawing, you should be very easily be able to follow the action and enjoy it.
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Conclusion -
So if you want to read just another fantasy setting manga with the classic tropes of power-ups, this is a decent read even till 20 chapters.