tl;dr I mostly repeat or develop the same point with more examples to make it across. Because my point still stands.
You are free to disagree, but you should at least understand why me, and apparently several others, think that the ending was cheap.
It means it's the entire point to it, and it's not that the author wrote herself into a corner and had to deus-ex-machina the hero out of it.
Yes, it is. A good author would have established the possibility of a power-up. (I'm discussing this in details below.)
Not establishing it
is the very definition of a Deux Ex.
And then when readers are surprised they complain about ass pulls because they didn't see it coming. Source: comments here.
They don't complain about the villain getting new power-ups.
First because the power-ups have been established before the MC shows up. That was the whole point of the fight against the tank's party. This was done remarkably well in comparison to the MC's power-up. We knew the fight wouldn't end without the MC, so everything before she arrives is setup, not resolution.
Second, the power-ups he did pull were consistent with the nature of his character, which is basically a collector of magic weapons... and their respective abilities. The only real "power-up" is the moment he introduces other weapons than the one he started with.
Third, most of the comments complain about one of two things. Neither is the villain getting power-ups but 1. the MC getting this random power-up out of nowhere and 2. the anticlimactic nature of this ending. Which is because the MC just pulled a random power-up out of nowhere. It's not surprise, it's just disappointment. And the MC needs to be established for the reasons I mentioned before and the ones I'll develop below.
And fourth, because the author took two chapters to create the illusion of an even fight and it was a complete lie. Breaking the mirror's power has precedent, so it works fine. Her one-sided beatdown of the "demon god" does not, because it was established as an even fight. I think I mentioned it before, but this would work ten times better without chapters 21 and 22. Remove them completely and this becomes good, because the tension is removed from the artificial "can she win against the demon?" back to "can she arrive before Tank dies?", where it was and was working pretty well.
But honestly, that's the point I don't really get. The very nature of the villain getting a power up shows that the hero will also get one. That's not a surprise. And sure, I'd still summarise it with "magic", but there are still enough explanations for it to not be a deus ex machina.
Yes. That's the common workflow. (Not an absolute rule, but it's a well-known one which works pretty often.)
But once again, the possibilities have to be established beforehand, not pulled out of nowhere. Otherwise, no stake, no tension... and an anticlimactic end to the fight.
If the character can use fire magic to light a torch, it's logical that he can power-up during a fight into setting his enemy on fire. It's not logical to power-up into conjuring a bubble of water around their opponent's head to drown them. Unless you established that he has untapped potential in other elements. Or you established that some rare magicians can awaken a second element. Many mangas do this kind of things when creating their world background. But if this kind of awakening happens after establishing that useable elements are set at birth without a hint that the MC is different in any way (reincarnation, unidentified element, rare title), then this awakening becomes a Deux Ex.
I mean, you're using extreme examples there that are obviously constructed to be off. Then again, I can think of at least one manga where it's true. First his sword started to level up from killing enemies. Nothing too big. Then it turned into a bow. Then into a gun. Then into a girl. Then into a motorbike. That manga's weird.
I think I know that manga. (Can remember the name, but tell me if what I say about it is wrong.)
If that's the one I think about, these upgrades were introduced before the fights where they become important.
These examples are not all that off by themselves. They are examples I took from other mangas.
And my point is not how ridiculous these things are by themselves, but how they are used without being introduced.
But let's continue.
In this example, the ability to change to a bow establishes that it's a shapeshifting weapon... and it doesn't come out of nowhere just as the MC needs a ranged weapon. From this point, we know that it will gain new forms as it levels up. Some shapes feels anachronistic (and even redundant), but it's not as outlandish as a new whole new feature being introduced mid-combat. When in a fight, the MC needs to use only the shape that were established. If they are insufficient as they are, he needs creativity or support. Not new random powers out of nowhere.
What I talked about was features introduced mid-fight, just as they are needed, without anything establishing even the possibility. I thought it was evident from the context, but apparently I was wrong. Sorry.
So, why does it feel cheap? Simple. As I said, no stake. The MC cannot be written into a corner and
there is no tension because we know a new power will be added whenever needed. The point of establishing powers before they are needed is that we know what the character can do, and has to work with. Better than pulling random powers out of thin air (staying civil here), a good writer will push characters to their known limits, then make them creatively use what they already have. A bad writer will escalate a situation to the point where the character has absolutely no way out, so something will happen without an established precedent.
For that matter, precedent doesn't always need to be established by the character themselves. Here, for example, if you want to establish that a character can power up due to "power of friendship" or anything even remotely similar, you can establish it through legends, or though a different character getting such a power up. Anything is better than literally making things up on the spot... and it's not even clear
what happened here. We're left trying to justify it ourselves without anything to work with. So we can only do exactly what the author does: pulling explanation out of thin air. (Still keeping civil, but it's tough.)
My own example will be Mash from... Mashle. He has one "power", which is superhuman strength. Granted, he can use it in insane ways, but he never gets new powers. It's just always strength. When backed in a corner, he either manages to power through with perseverance, or uses his body in a slightly ingenuous way (e.g. kicking the air at superspeed to hover). He never suddenly shoots lasers or conjure elements. And we know he trains constantly, so his limits are never clear but we know they exist. He has clearly established powers though their limits are fuzzy. He has clearly established character traits (mostly perseverance and friendliness). And yet, you can be surprised by they way he gets out of some predicaments. (Mostly because it's a full-on magic world, where the laws of physics are... more like guidelines.)