One immediate thing is age. The MC is technically over 15000 (!) years old when the novel begins, across 1000 (!) different worlds. Depending on the reliability of his memory, this would make him one of the most knowledgeable beings in literal existence. Now imagine someone with a cheat like that in a 9 year old body almost losing a duel to a 10 year old girl. You could write up all the excuses in the world, but it just sounds straight up silly. 

This next one is a me problem since other Xuanxia/Xuanhuan do this to some degree, but the current planet the MC is on is 100 times larger than Earth (I'm assuming diameter here like most people). Let alone being told that there are megacities that rival the size of Africa, the gravity on this planet must be utterly insane. It's difficult to suspend my disbelief in moments like these, just take the easy way out and do mystic portals that connect worlds together or something.

The prose narration has a habit of laying on thick amounts of exposition in a telling manner, sometimes to redundancy, which tends to bog down the pacing and make some sections feel like you got tricked into reading a lecture. Exposition is fine if one can time it right, space it appropriately, make it fun, or do all at once, but getting sidelined with undue worldbuilding homework feels bad, yo.

While having some side characters exist to prop up others happens in most novels, this story makes it a bit too obvious at times. Volume two is particularly guilty of this, where some Adventurers' sole existence was to get talked up by other mobs and then clowned on by the main cast, as a point to show the power scaling between parties or how much of a genius some youngster is. I'll admit face slapping is part of the genre, but it is too blasé and not built up well enough for my liking.

I didn't actually notice on the first go-through, but on second inspection I noticed that the dialogue and thought tags lacked commas or periods at the end of them, yet the question marks and exclamation points stayed. I'm not sure if that is a stylistic choice, but if so I find it kind of strange. There's also other grammar errors that you'll spot if you're not in skim mode, which can break immersion, but its not MTL tier bad.

Now while there's a fair bit to criticize, I think that under the hood there is potential. The premise of trying to beat an entity that destroys your soul each time you turn 15 is a neat concept, but going through the hoops 1000 times felt a bit excessive. There's also fun to be had with all the potential out-of-the-box skills someone with multiple worlds' experience could come up with under one particular world's system, among other things brought about with a bit of lateral thinking.