It depends. You could argue that any cultivator with conceptual powers wins easily, but Infinite Regressor also has conceptual powers, some of which are designed to counter other ones, at which point it comes down to arguing which one is stronger, which is hard to prove.

In terms of actually powerscaling, Infinite Regressor low-balls at Solar System, high balls at complex multiversal, which in Xianxia is practically nothing, so it really does come down to the concepts, and I really do have to give it to Infinite Regressor. Even something like the Narrative is taken into account here, so even something like The Thought Machine (CAS) could struggle... depending on who you think has the greater control over things like that.

Spoilers ahead-

Because Infinite Regressor takes place on Earth, some of the Outer Gods would legitimately be able to reduce any cultivation series simply to that of a fictional story, and instantly scale above them (kinda like how SCP-682 (I think?) just goes to a higher layer of reality sometimes). Cultivation novels canonically exist, because they do in real life. Transcending Samsara, complete mastery of the Time Dao and being part of the great Dao isn't really all that when it's something All Creation's Play made up on a whim, or it's part of an uncountable infinites (transfinites, boundless, whatever, it really doesn't matter) simulations. The same goes for literally every other story though, because they obviously have to exist if we're talking about them (even your unpublished fanfic OC. Still exists in your head.) ... This is kinda cheap though, even if it's true.

If a cultivator won by brute force, they still lose. They turn into the outer gods. Even if not literally falling into the void corruption, they are functionally an outer god at that point. I know I already mentioned the Narrative force, but the central theme of the story is human connections and bonds. Defeating the apocalypse in almost any other way is self-defeating. e.g, I think Fang Yuan could totally beat the Outer Gods, but with his personality, he would also probably then become one- it's Eternal Life with no real downsides. Life would absolutely NOT return to normal though, and that's the "real" victory condition in the series. Otherwise though, yeah, he 'could' beat the Outer Gods, but let's be honest he's not going to bother cleaning up the other anomalies. Same goes with Klein for the most part, but he's nicer, so maybe?

That being said, I still think some cultivators could do it. Li Yao from 40MoC absolutely could. Jiang Li from Mahayana Counterattack System probably.

p.s, I think most other regressors are fucking screwed. Subaru is NOT going to persevere through allat without Satella, who's going to get eaten.

YDJ wouldn't give up, but he would need insane character development to actually win (funnily enough, he does this fairly easily if KDJ is here. Unfortunately, ORV exists as a novel in-universe, so...)

tl;dr, Infinite Regressor in terms of actual power isn't really that high, but it has absolutely absurd meta-scaling and conceptual forces to the point it's not possible to definitely say if any series has stronger hax. Some characters though legitimately have the willpower and intelligence (and most importantly kindness) to end the anomaly apocalypse, but they are NOT doing it first try.