I never thought I would write a review, but here I am. I am writing this because too few people seem to get what this series is about.
First of all, I have to say that if you only like action-comedy shounens, this is not a series for you. If you want just a lot of fight scenes and plot twists, watch something else.
Now, let me explain why I liked Juuni Taisen.
Well, all the characters die in the expected order, and there aren't any major plot twists. In each episode, we get some backstory, a bit of fighting (I will talk about that later) and usually a death. However, I can say that the backstories are well written. Not even one of them tries to be too tragic, and the characters vary from pacifists to thieves and psychopaths. However, most of them contain some tragedy, and some of them really touched my heart. I even cried one, which I really didn't expect to happen when I was watching this series.
The action isn't really a focal part of this show, but the animation is really well done. However, I was more impressed by a thing you don't really see anywhere else. In general, there are two types of fights: the classic direct power clash and the more indirect fights in which the opponents try to analyze the opponent's unique power ( the activation conditions, limits and weaknesses) and try to find a smart way to win. Juuni Taisen looks at first like it belongs to the second class, but it has a surprising element of realism added to the battles: almost all of the plans DON'T work. And they end up failing for some really simple reasons that the characters just overlooked.
For some, it may sound anti-climactic, but I thought it was awesome how this anime showed just how quickly a life can end in battle.
Well, I don't really know much about music, but I enjoyed the op, and the rest of the music was pretty good. Not one of the best, but good.
I liked this anime from the beginning and around episode 10 I was thinking I might just give it a 10/11 (11 being the masterpieces). However, the last episode was just unsatisfying. Most of the people who are reading this are probably going to ignore my advice, but I am going to say it anyway: DON'T watch the last episode. Just pretend ep 11 is the conclusion. That is going to be much better. Or, at least, stop at the half point and imagine yourself what the winner asked for. I am going to say this again: DON'T watch the last episode till the end.
Overall, I give this anime a personal 9/11 and a recommendation to watch it only if you think you will enjoy it in the way that I described it.
You can watch it legally for free on Crunchyroll.