I went in having watched like 5 episodes of the first season of the anime years ago. With that as background I don't know why I should care about some of these characters. They realized this is an issue and thus did 1 min fight, 5 min flashback, fight continues, 5 min flashback, fight ends...
At the end when I realized this was the first of many movies I mentally started comparing it to the lord of the rings. I did not leave happy after making that comparison, this movie felt like it made no progress towards its stated goal of killing a super demon.
Overall I am just sad about the movie. So many good ideas (average soldiers saving stamina for the heavy hitters as an example) and animation I really enjoyed. Wasted on horrible overall writing. I assume they followed the manga too closely without having any idea if that is true. As a stand alone movie it fails since they give screen time to important characters that does nothing important in THIS movie.
Side note, if the default for new movies is >2h duration they should start with intermissions. Would make the movie theaters more money and likely make attendees happier as well.