I feel like the Hokage novels on Tomato have hardly any fight scenes, so I decided to write one myself. Since the protagonist can't obtain ninjutsu through normal means, some of the setting might be a bit off. If you can convince me, I'll make changes as you suggest (I'm easygoing).
The protagonist is pretty much invincible, though not truly invincible, since the power levels in the early and later parts of Hokage are too wildly different. So he'll still have to get stronger gradually. Besides, there's not much point in making him completely invincible. Readers who know the memorable parts of the story will recognize them; those who don't won't.
Two cheats: Pure chakra: All Kekkei Genkai and Kekkei Tōta have no requirements. As long as you understand the principles, you can reproduce them.
MAX affinity: As long as you don't show hostility, people generally won't act with malice.
