Well, it can be considered popular. It has a certain following. The world - building in 'ragna crimson light novel' is really appealing, which is one of the reasons for its popularity. Also, the character development is engaging, making readers want to keep reading to see how the characters grow and change throughout the story.
The male lead who killed Crimson Eye was Alan Yeager, and Aisders was one of the villains. In the manga, the scene where Aiders executed the male protagonist appeared in the 58th chapter, which was the battle between Alan Yaeger and the Saiyan after transforming. This battle was extremely intense and shocking.
Well, in an anime mashup novel, one key element is the blend of different anime universes. For example, characters from a shonen anime might interact with those from a magical girl anime. Another element is the plot which often has to be carefully crafted to make these different elements fit together. And the art style, even in written form, might be influenced by the distinct visual styles of the animes being mashed up.
Yes, there might be. Fanfiction platforms are filled with all kinds of creative crossovers. You can start by checking major fanfiction websites like Archive of Our Own or FanFiction.net. Just search for the keywords 'Naruto' and 'Ragna the Bloodedge' in their search bars, and you may find some interesting stories that combine these two characters in unique ways.