What pattern are you breaking?
In this case, you are hoping the accumulation of other people's writing clichés will carry your opening. You want to subvert the trope, but unfortunately this trope subversion is almost as cliché. It's used when the protagonist is a strong female, and it's used when the villain is a strong female. Maybe the only reader this would really surprise is someone who has never encountered any strong female characters, ever.
Redshirts emerged from an established pattern. It's a trope now, but it wasn't in the original Star Trek. Red shirts were the cop uniform or army uniform of the show. Disposable extras are obvious because TV budgets make them obvious. Any non-speaking character in a generic service uniform, paid to decorate the scene, is a Chekov's gun. This really isn't a narrative trope, it's a late-1960s television trope – the joke is aimed at a certain culture and a certain age group – maybe all trope subversions are, but this one isn't so fresh.
It's not that surprising
In a tournament plot 2 people fight, one always wins one always loses. This is 100% expected, in fact this is following the rules of every fight scene. It's like a coin toss, heads or tails, one or the other. An actual surprise is if the coin lands on its edge, or the coin rolls away into the gutter leaving them to work out their differences amicably. Or, as the coin is spinning in the air they are struck by a fiesta bus of drunk spring break co-eds. Surprise!
Maybe the biggest reason this won't surprise anyone is that I can't imagine how a story would be promoted or summarized without mentioning who the protagonist is. Joss Whedon's million-dollar idea was this exact trope reversal, and he even described it very similarly – a cheerleader alone in an alley chased by a vampire – but the name of the movie is Buffy: the Vampire Slayer so you'd have to wander into the wrong theater (and not glance at the marquee) to be surprised.
It's also problematic
Recast the scene with a man, as in April's answer. You have 2 characters in a dark alley, a human man and a drooling monster. Which one is going to be the surprise protagonist? You get zero points for guessing it will be the human being. For this "trope breaker" to be some sort of surprise to the reader, you are essentially relying on the reader's belief that any side-ponytail female is less than human. They would sooner believe there is no protagonist at all than see a vaguely human shaped vagina as an obvious protagonist. Dudes might fall for this. Really stupid dudes.
It's also patronizing, so I think it runs counter to the "pleasant" surprise. Consider some other patronizing reversals: a smart black guy, an educated Southern-accent bubba, a sexy librarian, a nice cop (or a bad cop depending on your cultural decade), an East Asian cabby that doesn't speak in pidgin….
The problem is these aren't characters these are just a 1-note joke. Once the trope is busted there is no further payoff. It's patronizing because it assumes everyone is a bigot. Hur hur. You've lost your non-bigot readers because they just cringed, and you lost the knuckle-draggers because they are rolling their eyes at your obvious SJW pandering.
Buffy: the Vampire Slayer lasted because it was satire. It went back to the same comedy well over and over with a healthy dose of suspension of disbelief. The hero characters remained formulaically shallow and misfit, but the villains were also anachronistic and funny, and everyone spoke in the same pithy dialog. They didn't just break that one cheerleader-as-victim trope, it became a smorgasbord of trope buffoonery. The show surprised us when it got serious, that's when they broke their own pattern.
For anyone who isn't stupid, you'll need a misdirection
April suggested separating the Magic Girl from the protagonist. The protagonist is surprised, even if the reader isn't entirely. The surprise reversal is still carried within the story. It doesn't rely on your reader being a sexist middle-aged baby boomer who grew up on 1960s boy adventure TV reruns.
Establish an in-story pattern first by showing the monster attacking other side-ponytail women. Okay, not quite so literal, but the idea is to not make this the opening scene. Build up the monster as something that kills and kills successfully. Raise the stakes by letting us meet the magic girl in her normal form. Does she have amnesia or a secret identity? Did she come to this planet to get away from monsters? Whatever the pretext, we should see her in her normal world – and also the monster in its normal world, killing – so the reader has expectations about these characters in particular, not just the surface situation (a tournament fight in an alley). Now when the reveal happens, it breaks your established pattern.
Use a decoy character who looks more like a hero and kill him off first. Similarly, you can have a few characters so it looks like an ensemble. Surprise, the hero is Beth the 30-something grocery store clerk, or the burrito waitress – anything that gets your character count above 1 monster and 1 human because most people will see there're only 2 characters and pick the human-shaped one, despite that pesky un-heroic vagina – we'll just assume that's a character flaw that she'll get over somehow.
She dies. Sort of like having your cake and eating it, the monster kills her before she comes back as laser girl. Maybe this is how she loses the amnesia, or maybe she is with friends (who must die before she can reveal her true form?). It's going to open more questions than it answers, but we've turned a 1-note trope reversal into an origins mystery. Again the readers may not be entirely fooled but the element of surprise takes place in-story. There might be consequences from not-dying if there are friends/witnesses, plus there's a savior/resurrection theme and it could explain any other transformations (assuming she stops dressing like a 30-something suburban mom).
Lean further in to the subversion. Here's the opposite. Yes, she happens to be LaserGirl the Invincible but she left all that behind to be a suburban mom, dammit! Similar to Bewitched and other immortals among us characters, the hero is revealed to be an unstable balancing act between maintaining her chosen lifestyle and saving the planet. She doesn't have to be a reluctant hero – she's actually darn good at the hero part. It's the PTA meetings, and dinner at 6:30, and hiding the laser powers from the husband and kids, and this neighborhood has no parking…. You still get your opening scene switcharoo, but it pays off longer as cracks in a mask.
Hit that joke hard in Scene 1 and keep hitting it forever. Establish the main character as the victim and kill her off. Then, an alien warrior (or whatever) inhabits her dead body to fight monsters, but the 30-something mom's memories persist. She's got a mish-mosh jumble of overlapping memories and character traits. It's probably been done a hundred times so it's all in the details. Like Buffy, you get to tell the same kinds of jokes over and over through an iconic self-contrasting character. In Hero-mode she has quirks of Mom, and in Mom-mode she has quirks of the Hero. You can swing her in the opposite direction as needed to contrast whatever is going on (as opposed to Cap'n Ms Marvel who is 2 of the same hero archetype, so why get her memories back she's the same character either way).
It depends on the stakes you establish in the story
There's a whole character behind this trope. Was she pretending because she wants to attract monsters, or was she on her way to spinning class at the gym and a monster found her? Is she more interesting as a normal woman who discovers she has powers, or as a powerful being who wants to be normal? It depends on what sort of themes you want, or what you want this character to say.
Obviously, if all she has to say is "BIFF! BOOM! POW!" or maybe in this case "ZAPP! ZAPP!" then you won't need to over think it. You are trying to raise the stakes with some choreography (running, fighting) and then a deus ex machina. As it all happens in the 1st scene it will be tough to make us feel anything much. Here's a normal lady, but nope she isn't. There's a monster, but it died immediately so I guess it wasn't very threatening.
How does this pay off in the long run? The next monster might fall for it, but the reader certainly won't. This is a 1-note "opening twist" and it's gone in a split second. Now what? How can this scene pay off later in the story?
I think what worked with Buffy is the athleticism shared by cheerleader and warrior. Her personality is iconoclast, but she still did backflips and gymnastics like every other female warrior. If this trope-reversal is to develop into a character trait, you'll want to consider something that moms and monster-killers have in common where this character can find her niche.