[THIS REVIEW CONTAINS VERY MILD SPOILER]

First off, to give you a quick summary, I pretty much enjoy this manga overall, but definitely not the opening. Right from the first chapter, you can see it doesn’t have a good hook. Things even slowed down for the next 3 or 4 chapters, but I believe this buildup is necessary for the story to take off later. If you stick to around 25 chapters and you still don't like it, I say drop it. My overall enjoyment: 8.8/10 (Fun read). Re-reading value: 6/10. (Suspense is what keeps this manga good, by re-reading you've known all the details). Anyway, here's my detailed review:

1/Opening: 7.5/10: Can be a little misleading. Without reading the genres or seeing the cover first, you would think this is a basketball romance manga after several pages, and I don’t blame you. However, toward the second half of the first chapter, you will realize that uh oh, things are getting tense! However, this opening is cliche at best, with monsters appear and kill everyone, seems like a scene taken straight out of Godzilla
Not only is the opening not hooking, but it also drags on for a little while. This has both pluses and minuses. On one side, it is necessary to depict the terror and ignorance of the survivors of the situation, of what is going on, and their quest to gather forces. On the other, 75% of the first 5 chapters is just monsters destroying building, after a while it gets repetitive, which might lead to high rates of dropping (Remember, the first few chapters are crucial as to whether a reader drops a manga or not). Furthermore, the premise of the manga is not stated explicitly until many many more chapters: what are these creatures, where did they come from, how to stop them, etc. If you stay with it, it could be rewarding though.

2/Plot: 8.8/10: It is primarily a survivor manga. Thus, it shares many characteristics with the likes of Gantz, but it also has a few uniqueness.
Similarities include: the suspense, the killing of characters, and several internal conflicts, which give way to character development.
Differences include: The characters don’t pick up information and intelligence as they progress in the game. In Gantz, the players have to find out for themselves the functions of the armor, how to win, etc., in Tenkuu Shinpan, the players have to explore the functions of the masks, and learn about the powers of the “Close ones to God” ALONG the way, which makes for an interesting “adventure,” so to speak. In Hakaijuu, the characters just run and fight, not until VERY later on did everything get laid out ALL AT ONCE, BY AN AUTHORITY, which I find not as fun.
Pros:
+ The fact that it kept you in the dark for a really REALLY long time. It gives the readers the feeling of actually being the manga characters: fear, ignorance of what’s going on, etc.
+ It gives you a rainbow-after-the-storm vibe, you know, all the couples take off and the happy-feel-good motif. However, to be honest, it is not really detailed. I’m ambivalent about this. (more under character development)
+ The most prominent feature of this manga is the 3 “breaks,” or “intersections.” If you have read Kamisama no Iutoori parts 1&2, or 20th Century Boys, or Code Geass Parts 1&2, you’ll know what I’m talking about. The story temporarily stops with a cliffhanger, and goes on to depict a whole other frontier, with new characters and everything (Code Geass). Or it ends with a “Great Battle,” and doesn’t show the aftermath, but rather flashes forward to what happens after it (20th Century Boys). It is not until very later that these 2 “seemingly parallel” theaters converge, and both sides start trading information, and what happened is actually revealed. This is extremely satisfying to me for some reasons. Not the best way to put it, I’m sorry, but I’m sure you’ll get it when you read it. All the mangas I reference have 1 of these, this manga has 3!!! All of which are well crafted, which shot this manga way up in my list.
My dislikes:
+ 50% of the mangas is just the author showing off his drawing skills of monsters and buildings. It is beautiful, but after a while it gets really repetitive and boring.
+ The author uses the same motif over and over again: the characters get exponentially stronger, while the monsters get EVEN MORE exponentially stronger. Imagine Dragon Ball Z. The difference between SS4 Goku and Goku is achieved in 500 chapters. Now shorten it by 15 times! You get this manga and its progression of power. Thank God he doesn't use the power of hope and friendship!
+ The author recycles portrayal of characters (more under art)
+ Mediocre re-reading quality. It is a fun ride, but it’s definitely not half as exhilarating as the first time, since you know all the “breaks.” Cherish your first time, fellas.

3/Art: 8.5/10: the art is good from the beginning. The building and every monster is drawn very detailed (although the monsters can be a jumbled mess and indistinguishable sometimes). Most impressive are the armors and technology. Looking dope as hell! However, what I hate most is that the author recycles his portrayal: he draws immediate family members exactly identical (mom looks exactly like daughter, dad looks exactly like son). This is the biggest turn-off for me. At least change something yo!

4/Character development: 6.5/10: The biggest disappointment for me. This story features very limited character development, even for the protagonists. While other survival mangas focus on the change in personalities of characters after facing death, this one focuses too much on fighting and action. The author should have cut down on his monsters and build more onto his characters. The romance develops very subtly, it’s just like “boom, we’ve been together since day 1 after so many battles, so I guess I’ve fallen in love with you at some point.” There’s very few moments of introspection, you know, when the characters just stop to realize and reflect on the journey and their feelings and all. You see none of that here. Also, in other survival mangas, the MC usually man ups and resolves to fight when something tragic happens to him (love interest dies, friend dies, shame of not being able to protect anyone, blah blah). In this manga, however, the guy just become totally badass overnight. Biggest turnoff for me so far. In short, you don’t see the roads (moments of introspection, shocking deaths), you only see the destination (couples just take off, MC just become super brave overnight).

5/Suspense, thriller, and buildup: 9/10. Extremely good due to the fact that we are kept totally in the dark. We ask ourselves wtf is going on. The battles are extravagant, and I usually get a Star Wars vibe at the big battles!

6/Ending: 8.5/10: This is not a great ending or an ok one, it’s somewhere in the middle. High in the spectrum. The finale has a big twist which I really like. I won’t spoil it. It also sees a big change in a character, but to some it may be entirely bullshit and cliche and plot armor, but again I really enjoy. Overall, as a Shounen, it is obvious that a happy ending is expected. It satisfies the typical ending and ends on a high note, giving its readers a feel-good vibes. It provides proper closure, as well as the “look forward to the future” and the “people marry, live happily ever after,” and it does so in a not so cheesy way, but a lighthearted way which leaves me feeling kinda bittersweet (I guess?).

Note:
+ If you finish this series, and are crazy about it, you can always shoot me a message and have a friend to discuss it over with! I love this manga but (somehow) it is not popular, so the need to discuss it off my chest is real!
+ There is no full English scanlations of this series so far as I’m concerned. I read this in my native tongue, which is Vietnamese, in case you were wondering how I managed to finish this.