I was thinking about this one. Could Starlight actually take down Flurry? But then I remembered some specific details. Starlight has; removed the cutie mark of an alicorn and her friends using a spell she made, messed with the timeline using a spell she made, almost killed discord, and she almost would’ve defeated chrysalis on her own if it wasn’t for the fact chrysalis was empowered by grogar’s bell. Saying all that? Yeah, I think this is possible.
It’s worth noting she explicitly stated that Star Swirl did the hard part for her, and the Map itself was a notable component of doing that one.
It wasn’t difficult to change Star Swirl’s spell. He’d already done the hard part. But figuring out I could use the map to go to any time or place and pull you along with me? [short laugh] I even impressed myself with that.
“Almost killed Discord” is also not actually a feat worth too much merit, considering: he baited her into doing it to look unhinged, it was said to be a banishing spell (they don’t talk as if he died), he let it work, and he effortlessly circumvented it as if it weren’t there anyway, that is to say it did absolutely nothing to restrain his powers. The S6 finale, narratively, unlike Starlight, treated Discord like an instawin condition, if he could’ve used his magic against Chrysalis and her army.
She was also outmatched by an alicorn Cozy in terms of raw power, and to tie into the above note about Discord, she by some unknown means lost to Sombra offscreen (could’ve been a fight, could’ve not), whereas Discord could’ve always beaten him instantly. Starlight never, at any point, got through one of Twilight’s shields, and it’s extremely plausible it’d be the same case for Shining Armor.
Flurry, while blank slate-ish, from what we can gather, has a lot of raw power, would probably gain a good amount of finesse naturally, and can be reasonably assumed to be quite intelligent based on her portrayals (see A Flurry of Emotions, for instance). Her parents, if they taught her anything, aren’t exactly slouches in combat skill, though this is ironically a bit more readily apparent with Cadance in canon.
Long story short, it’s probably believable that Starlight could beat Flurry Heart by some means. She’s got quick reflexes, is smart, is powerful, has good versatility, and would have decades of experience over a teenage/young adult Flurry Heart, but it wouldn’t necessarily be easy/a victory taken without a scratch.
Realistically, I think Starlight’d use cutie mark magic as a last resort if she would be willing. She’d only switched Celestia and Luna’s cutie marks as a panicked measure, and regretted it pretty much immediately, and never tried to use that removal spell post reformation otherwise.
If you omit a LOT of details. Then yes. It may seem that all these merits are something beyond natural. But if you think a little, her power level will drop many times over. But experience nevertheless is, and very powerful magic. Defeating an inexperienced young alicorn will not be very difficult.
Edited