If some of the floors weren't extremely small and Volume 21 didn't appear to severely limit Velgrynd's destructive capacity at the solar system level, we may get a likely rating for this, but unfortunately we can't assume that right now.
Well, to be specific, the floors themselves aren't really important since Ramiris can extend the space to however much she wants. We see examples of that from the fact that there are stars and day-night cycles in some floors.
Plus some floors are just like that. Why would there be stars in a maze that's supposed to be just a completely closed room?
Plus that capacity of Velgrynd clearly referred to the cardinal world, where magicules exist. We already know magicules can, overtime, strengthen anything that's exposed to them to insane degrees.
That was made clear in V17:
At this point, Velgrynd, unlike Veldora, could maintain a steady cycle of magicules entirely within her own body.
She didn’t need to replenish them from the air, and they were never expelled from her body. That was a skill she’d learned as she traveled through all the diverse worlds she had seen. As a result, she wasn’t paying attention to this world’s magicule concentration, so she hadn’t picked up on that fact yet.
It was normally impossible, after all, to go back the way you’d come when traversing worlds. Even if you had an Underworld Gate handy, you’d still be limited by the size of the portal. Someone like Velgrynd, who could use her Dimensional Leap skill with no limitations, was a blatant exception to the laws of nature.
Therefore, compared to the magicule-rich environment in which Velgrynd was born, the standards for “strength” in this world were quite low. She did not have to wait long to see this for herself.
OTL V17C2
Gensei and Kondo were pretty much equal in strength—but that strength was far below what Kondo had when she’d met him. When humans in physical space crosses between worlds, most will die from the resulting exposure to concentrated magicules. On rare occasions, however, the magicules can break down and reconstruct their bodies, making these humans tougher and more resilient.
OTL V17C2
“Don’t get so full of yourself, Delia! This world exists in physical space, full of possibility. With the right number of magicules, things can change in every which way. We yohma only become complete once we obtain physical bodies. I want you to make sure you don’t forget that!”
OTL V17C2
And this affects the Laws of said world as well:
Delia, after that tirade, realized he was right. This world was inferior in terms of pure strength, but that was only because its physical laws differed from the ones they were familiar with. She admonished herself not to forget where she stood until this invasion was over.
OTL V17C2
And that also means less impact on the laws themselves due to less magicule concentration, and thus weaker magic.
Her mind was bursting with optimism. And she was right—magic in this world was more the realm of offensive sorcery and energy-sapping curses, and it was hard to cast spells that could directly rejigger the physical laws of the world. But praising Ludora for this was going overboard. The Oharu of this realm had no such powers; all of this was better chalked up to mere coincidence. To put it bluntly, the mystics had luck going against them. The moment Velgrynd had begun to play an active role, their invasion was all but doomed.
OTL V17C2
Because it was created by Veldanava using his (special) power:
As Guy explained, this planet was created through the power of Veldanava and therefore wasn’t going to be catastrophically torn apart or anything. But Milim’s power was still rising, and if left unchecked, she’d encase the whole planet in a thick layer of contaminating magicules.
…Well, if there’s still gonna be a planet at the end of the day, then great. And I guess it made sense. After all, even Carrera’s magic wasn’t the sort you should really use on the surface of a planet. We only got away with it because we were on a world that could withstand such lethal force. Otherwise, at best, some attack from her would’ve bent the axis of the planet by then.
OTL V20
The Divine Tree does the same:
Ciel’s being so vague about this, I suppose the strategy had little chance of success. Frankly, it would have been crazy to go through with it.
But if that was the case, there was no point in worrying about the “why” of it. What about the “sacred tree” Feldway set as Milim’s goal?
This is a divine tree that protects the capital of the Sorcerous Dynasty of Thalion. It is a tree so large it can hold a giant city. It also seems to play the role of stabilizing the magicules in this world, preventing natural disasters. (Ciel speaking)
OTL V20
Zarario looked behind him. There was the sacred tree, standing in this land since time immemorial. Rooted deep in the earth, it protected the planet from all kinds of natural disasters—and it was still standing strong, even after taking all of Jahil’s flames. It is the same tree that kept the past Milim/Guy battle from being any more damaging.
OTL V20
But if you looked at it another way, that was the only damage that resulted last time. This world was protected by gods who put up many obstacles to destroying it. Guy, the great Arbitrator, was the best example— feared by mankind as the strongest and most terrifying of demon lords, but still constantly protecting this world, as per his pact with Veldanava.
Several beings helped him with this role. There were the demon lords selected by Guy, including Ramiris, the other Arbitrator. There were the Heroes, their nemeses and counterparts—and now there was Masayuki, the reincarnation of the most powerful Hero there ever was. Velgrynd was with him, too.
Apart from these, there were relics like Skyspire Tower, influenced directly by the hand of these gods, which also offered the people their protection. The sacred tree was another such relic.
OTL V20
That's also why Feldway ordered Milim to destroy the divine tree first, so he could actually destroy the universe himself.
“Tch! You came sooner than I thought. But it’s too late. Milim! Use your power to shatter the sacred tree in our way!”
OTL V20
His rule over Milim was likely too unstable for him to focus on anything else. Or perhaps he wanted to prioritize the destruction of the sacred tree.
OTL V20
Feldway wanted to revive Veldanava, but he failed after losing Michael. If so, what he’ll want next is…
Zarario felt a chill run down his spine. Maybe he would want to destroy this world.
There were only two holy relics left in this world: the sacred tree and Skyspire Tower. There was also the labyrinth created by Ramiris, but that should be considered its own separate category.
Feldway himself couldn't destroy the Divine Tree so he made Milim do it:
“M-me…?” Veldora sounded terribly aggrieved by this, but he was ignored. “
So once this darkness goes away, you’re going over to the divine tree.”
“Wait, what?!” Veldora balked.
“Yes, the divine tree. Feldway probably wants Milim to destroy it.”
Feldway had failed at it once, but he hadn’t given up yet. Now that he had regained full control over Milim, he had no reason to sit idle. And that was the “thorny problem” Velgrynd had in mind—Milim and her group were on their way to their next destination.
Between destroying the gate and trying to destroy the divine tree, there was just one thing Feldway could be aiming for. He had no intention of respecting anyone else’s territory from the get-go—or really, he had no reason to. If this planet was wiped out, all territorial disputes would cease to matter
Even the characters protect the planet from barriers:
I had seen my fill on the way. The only reason this planet was still safe was because of Velgrynd offering to help. She was worried the planet would be in a state of chaos if things continued as they were, so she came all the way over for me. I supposed Velgrynd wouldn’t like it much if this world fell apart, either, so I didn’t really need to thank her that much—but then, if it did, she could just do one of her Trans-Dimensional Leaps, right?
Not that I was gonna turn down her help, but… She told me she’d left a Separate Body by Masayuki’s side, so the Velgrynd I saw contained just 70 percent of her power. Even so, it was helping me enough, and I was relying hard on it. I suppose that’s why I dared to ask Velgrynd: “Hey, not to get too greedy, but is there any way you might be able to keep that sacred tree from getting damaged?” The results were about what I figured.
“You’re as silly as ever, aren’t you? Milim isn’t even trying, but I’m working my hardest keeping a Star Barrier going, you realize. And even then, she’s still causing tons of damage. She can work directly with stellar particles, you know. And there’s no such thing in this world as a barrier that can absorb a Drago-Nova head-on.”
And yet, the Quartet Skill was SOOO strong that it would bypass ALL of that and still destroy the planet along with the solar system.
The whole point of the verse's scaling being reduced to below tier 2 just because of some statements that limit the character's AP
BUT are in context to a planet that itself is harder to destroy than a whole world/universe baffles me lmao.