It depends on your language proficiency. If you have a basic understanding of Japanese grammar and vocabulary, maybe N3 or N4 could be enough. But for a smooth reading, N2 or above would be better.
To read The Shield Hero novels comfortably, you'd likely need at least an N2 level in the JPLT. This assumes you have a good grasp of kanji, complex sentence structures, and a wide vocabulary. However, if you're willing to look up words and study as you go, N3 could also work, but it might be a bit challenging.
The ideal JPLT level for The Shield Hero novels is N2. With this level, you'll be able to handle the language complexity and nuances in the text. But if you're a dedicated learner and don't mind using reference materials, N3 could still allow you to enjoy the story to some extent.