It's seven samurai in space with bits and pieces of warhammer nonsense. It's basically a movie for children.
I also found it unrealistic that somebody so young would have already fought "countless battles".
I think the movie suffers from overly liberal use of creative license. Sure, I t’s science fiction but there needs to be some sense in most of a movie’s events. For example, why did a hulking huge space based invasion force need to show up to a ground village of a few, basically, organic farmers? I figure Zack thought it’d look cool. And it does, but the story becomes less believable. I can’t help be feel that a ‘high quality’ team of creative writers should have been able to make that particular event work and make sense. I think this is a common problem in a lot of movies that try to be bigger than they are.