I'm gonna assume other fanworks did the idea of having a fair fight between youkai and human or something of that nature
cause last one i remember doing it was Touhou Reireimu
It's taken from Perfect Memento in Strict Sense's article on Reimu:
Without a way for any youkai to defeat the Hakurei shrine maiden, many youkai felt that their meaning to exist had disappeared, but by taking some ground-breaking measures she solved this problem. That is, the introduction of the spell card rules. [...] Following these rules allows for duels that feel almost like sport. With the introduction of these rules, it became possible for even relatively weak humans to challenge youkai. At the same time, it became possible for youkai to freely defeat the shrine maiden without worrying about the consequences.
However, Perfect Memento is not infallible. It's written by Akyuu, and Akyuu can be mistaken, which allows doujin authors to hew extremely close to canon and still depart from it in certain ways. For example, by having the spell card rules be effectively Yukari's idea, inspired by Reimu's habit of naming and loudly proclaiming her attacks.
Another major way this doujin does depart from Perfect Memento, in a way that's even rare in fanworks, is including Reimu in the group of "relatively weak humans", and having her not just lose to, but almost
get killed by Rumia in the first chapter. That part is not in line with Akyuu's description of her at all:
Out of the generations of shrine maidens, her sense of danger is the most lacking and she has meager training, yet her power is considerable. She's exterminated multitudes of youkai with her keen intuition and her natural good luck.
So far in SPELL Reimu has done almost nothing BUT train, with Yukari taking the reins. This is still very much covered by artistic license, however - Akyuu cannot write about what she doesn't know. So there are no contradictions as long as Yukari training Reimu stays a secret.
What will be a bit harder to account for is Reimu "lacking a sense of danger". In SPELL, she was taught a sense of danger in the harshest manner (by almost being killed stone dead), and is already a much more conscientious character than Akyuu's account reflects.
Reimu being naturally OP and disliking training is usually a staple in her duality with Marisa, who wasn't born with any abilities safe for those she acquired through hard work and learning. I'm curious how this is going to be handled in SPELL.