This is the story of a Kusa who gets stranded in Gensokyo and grows stronger in another world.
(The following is the synopsis for Volume 3.)
Kotomine Shirou: Scheherazade, could you do me a favor?
Scheherazade: Please, Master.
Kotomine Shirou: (a holy smile) Could you please move your legs off my waist?
Emiya Illya: Arjuna, go! Shoot down that shiny guy over there!
Arjuna: …
Karen: Big Brother, I'm here to help!
Ramesses II: Little lights, bask in the radiance of the sun god Ra!
Cao Ying: Sigh, why is my Holy Grail War such a mess? What do you think, Lancer?
Kiyohime: Mm, Master, why did you stop?…
(The first dozen or so chapters of this book have some questionable content, so I recommend starting from Chapter 20. Also, it really does get wholesome; see the title of Chapter 43.)