It's nice to see an action packed recommendation for once, and I loved the fight scenes. The artwork got a bit thin at times (limbs like bones, OMG!), but that's a small issue~
Youmu as usual is the never-give-up-type, and I lolled hard when she actually lost. XD
I never like power level in Touhou, actually. But one can tell that Sakuya is obviously stronger than Youmoe. Heck, she is one of the most haxxxed character in Touhou as well.
Spoiler:
But that doujin is kinda different. It's so refreshing to see Sakuya's "other serious" side that we rarely saw in other works. You can see at her pasts that vaguely described there. Somehow, "past-her" looks like Saya from Saya no Uta, lol.