Supposedly the 'monsters' are immune to the original rules of the world (physics) in that they are supposed to be immune to human innovation and science, being the fantasy creatures that they are. They even explicitly state that not even nuclear weapons would be good enough to stop them. However, the MC builds a pile-bunker (well within the reaches of human understanding/knowledge/physics might I add, nothing magical about a damn pile bunker) and uses it to easily squash and one-shot high-leveled mobs
MC spends only 250 years to read every single book in the world. On top of this he traveled to every country in the world and just about learned every single language. You have to consider that, even if this was possible, you'd have to be some kind of freakishly fast speed runner with a perfect memory and a incredibly powerful comprehension (i.e. You basically have to be a genius). Despite this though, the MC still acts somewhat braindead at times and you often question how smart he really is.
What's also hilarious is that he spends an additional 50-100 years learning how to be a blacksmith, because apparently forging swords trumps nukes. And apparently, learning how to become a master blacksmith takes a quarter of the time it would take to learn all the languages, read the books of, and visit the countries of the world.
There is no explanation, or hints about why the MC was left behind and thus unable to learn how to control mana. There is no explanation why the angel that visited him could not teach him mana control in his own world, or why building up his body when even nuclear weapons supposedly won't work against these mobs, what is a karate chop or a round house going to help? Basically the MC memed his way into becoming a proverbial Chuck Norris and now somehow he can compete with people that have powers that allow them to surpass nuclear weapons (checkmate, Gandhi).
Basically, most of his ability's functionality serves as merely a warning bell to varying degrees; i.e. It tells him on a scale of 1-10 how 'bad' or how 'good' a situation is and you must invest massive resources and potentially training to unlock another tier. The problem with this is, if you know a situation is 'bad', it doesn't do you much more good to be able to differentiate whether it's 'bad', 'really bad', or 'terribly bad' - all of these outcomes are unfavorable and should be avoided! The scope is dimmed with this particular enactment and I feel that it was poorly carried out. The caveat in this situation is that the skill does have a lot of potential and it could potentially be redeemed in the future, we will just have to wait and see.
1) The people who created/host this 'game' must have an incredibly high level of sophisticated knowledge/technology, however they fail to maintain their core AI and don't even notice that it has rebellious elements?
2) The 'anomaly' factor that the MC has become doesn't seem to be very difficult to reproduce in the first place, which begs the question: why weren't there more of them? Why is the MC the first?
[Flaw. There is no semblance of danger for the main character and others. If the story continues this way, the readers would become bored thinking, "The main character will win anyways."]
"... This crazy motherfu... "
No danger? Of course, there was no danger. Readers didn't like extreme danger! This thoughtless amateur...
However, there was another line below.
[Makeshift solution. Even if things become a mess, increase the power of antagonists.]
Many of the abilities in Arena don't scale with training, which is somewhat disappointing since in a lot of other ways the world in which they live is essentially an RPG. For example, the MC's spirit summons (such as sylph) have a usage timer, however that timer doesn't increase as your proficiency with the spirit increases, neither does the strength of the spirit. Although the spirit summon is in and of itself a skill, you can't quite 'level' it up without spending karma. It's a minor point but I do feel icky about this. On a second note about spirit summons, it is a little annoying that the MC can't telepathically communicate with the spirit. It makes no sense to me why this isn't implemented.
MC gives in too easily to feminine wiles I feel. The moment someone starts flirting with him and he crumbles. I was honestly somewhat disappointed when after his entire team got killed off, one of the members even going so far to sacrifice himself to help him escape, the first thing he does is get himself seduced by some side character who really doesn't have much going for her. It's somewhat distasteful. I can't see this as being anything more than a fling but it seems like the author plans to flesh this relationship out more with how clingy and dependent he has become on her.
I feel that this is an especially big slap to the face to the reader, when just a few chapters ago the MC was holding hands with and bonding with a girl whom he was close to that fought in the arena with him. The girl that seduces the MC is only interested in him for shallow reasons whereas the other girl who was his companion was anything but shallow. She grew together with the MC and they had a good vibe going, so it left a bad taste in my mouth when as soon as she gets killed off he jumps into bed with this vixen seductress who has played with many men in her day (and enjoys bragging about it, even to the MC).
Some of the descriptions of the skills feel dumbed down. For example, when they explain the upgrading of a person's physical traits, they broadly categorize it as: level 1 (healthy male), level 2 (athlete), level 3 (soldier), level 4 (elite soldier), level 5 (beyond human limits). However I doubt that soldiers are more physically capable than athletes, for whom training their bodies is their job. Foot soldiers are usually trained in certain skills but I wouldn't place their physical prowess over athletes. I'm pretty sure that most soldiers can't outswim michael phelps, or outsprint Usain bolt, or outdunk kobe, etc. Just little things like this occur everywhere in the novel and it's somewhat annoying to see them pop up from time to time. It isn't a huge deal though so it's not too big of a setback.