As much as I like Touhou, I can't help but laugh at you. To defeat a Chaos God, you have to eradicate one of the facets of human nature. Not just for humans either, for every living thing. You have to erase the desire to kill, for whatever reason, to defeat Khorne. You have to erase the desire in sentient beings for pleasure, decadence and hedonism, to defeat Slaanesh. You have to remove ambition, planning, dreams, goals, and scheming, and greed to defeat Tzeentch. You have to erase the laws of nature to defeat Nurgle.
Reimu would never be able to defeat a Chaos God, because she, like every other human, caries some Chaos within her soul. Her very personality is that of Chaos. Chaos gods aren't "gods." They are emotions. Raw emotions that every human and sentient alien possesses. In the warp, these emotions become manifest as the Chaos Gods, and all of their daemons.
Reimu can't defeat them, because to kill a Chaos God, you have to remove its emotions from yourself, and also from every other living thing in the universe.