One interesting 'death battle reaction fanfiction' story could be where characters from different universes react to a fight they never expected. For example, superheroes from Marvel reacting to a battle between anime characters. They might be shocked at the unique powers and fighting styles.
Yes, unfortunately, Hange met their end in the manga. It was a poignant moment for fans. The circumstances leading to their death were quite intense and had a significant impact on the story.
First, know your characters well. If you're having characters from different franchises react, you need to understand their personalities, values, and how they would typically respond to a violent or intense situation. For example, a pacifist character would react very differently from a battle - hungry one.