It's a little on the short side for my taste, but it's pretty well written. Barely any grammatical mistakes aside from one or two typos. My main suggestion for future stories would be to include more of the victim's reactions; what is she feeling? Does it hurt? What is she thinking? Aside from Starlight's eyes widening twice this story doesn't really contain anything like that, which means the whole thing is mainly a mechanical description of the order in which bits are removed, which isn't hugely interesting in its own right. As with almost all good stories, a good guro story requires character development. Does Starlight struggle to break free, fighting Twilight until her last breath? Does she accept her fate? Does she try to plead with Twilight to release her? What does she think about when she's been reduced to a brain in a jar? Is she plotting revenge? Is she thinking about how her life might have turned out different if she had accepted the main six's offer of friendship? As a general guideline, readers should be able to get a sense of someone's character even if they don't know any of that character's backstory.

All in all, though, I'd say this is a fairly decent first effort, and I certainly wouldn't mind reading more of your work in the future.