Your writing may or may not ever involve the sort of catastrophic showdowns seen in video-game boss fights, and if it doesn’t then this certainly isn’t going to be an issue for you. Remember Achilles in the Iliad, the fellow who cut a bloody swath through the Trojan armies for years, only to die in the penultimate battle of the war because a wussy brat shot him in the heel? Try not to do that, it reads as a very clear Deus Ex Machina.
Background info- this is Sundowner (all credit to the Metal Gear Wiki for the screenshot):
A towering Arkansan cyborg with a rattler embossed on his chest, twin high-frequency machetes capable of bisecting pretty much anything, superhuman speed, strength and reflexes, and an utterly insatiable lust for blood. He’s the mid-game boss of Metal Gear Rising: Revengeance (which remains my favorite moronically glorious game name,) a total hardass who sees war as an inseparable part of human nature and loves doing his part to keep it that way. He’s cartoonishly evil, yes, but you can’t say he isn’t tough. He curb-stomps everyone he runs into, prior to Raiden (your character in-game.)
At which point you’ll dismantle him in three minutes or less if you’re much good at Rising’s frenetic cyborg slaughterfest, probably taking minimal damage and certainly not feeling very impressed. I’m sure you understand already on those grounds what the Sundowner Effect is, but humor me while I over-clarify. If there’s a Sundowner in your writing, it’s not in quite the same way as in MGR; the weakness of your ‘boss’ character comes from the way your wrote him or her, not a set of mistakes or misjudgments in the way the boss fight works in a game. That aside, you won’t have to work particularly hard to find your Sundowners. You’ve made a Sundowner every time you hype up a villain or antagonist for an extended period of time, then immediately knock them over like a particularly flaccid cardboard cutout during the showdown with your hero. For example:
Camille sighted down the rapier, checking it for bends, chips, any signs of tampering. Nothing.
“If you’re finished wasting my time, let’s begin.” The Marquis removed his hat and wig with one swipe of his hand, flicking them off to the side. “With you put in your place, I can attend to less pitiable company.”
He gave a lazy salute with his own blade and started forward. For a few seconds they exchanged thrusts and parries. The Marquis was good, but not as good as he liked to think. Camille soon had him constantly on the run, backing him up over the hard-packed dirt of the frozen fields. Camille fainted for his left knee; he took the bait, and so too the point just under his jaw. He twitched, clutched at the blade twice, and collapsed.
I’m not going to provide you with examples of the hyping-up end, you already know what that looks like- the Marquis’ name muffles any conversation, pales great swordsmen throughout Normandy, etc. Any fight in which your heroine is supposed to face an extremely powerful opponent should express that. It should be brutal, physical, and above all, try to make readers forget that they’re reading the heroine’s perspective, and she’s probably not going to die halfway through the book. Something a bit more like this:
The Marquis snapped his rapier up through a salute and back down to center line, where it stopped instantly.
His control is nearly perfect, Camille noted, trying to fight off the little voice in her head- or maybe it was her gut- shrieking Mistake!
Her split-second’s introspection almost killed her; the Marquis lunged forward so quickly his sword was inches from Camille’s temple when her reflexes kicked in and she darted backwards, flicking at the attack. The Marquis allowed a tiny smile to crease his weathered lips. He dropped the point of his rapier for an instant; Camille sprang forward instinctively, before she had time to see the obvious trap. The stories all described the cold kiss of steel on skin first, but she felt only tearing pain along her back. Somehow, she managed to deflect several strikes through the haze in her eyes, and when it cleared, she saw the Marquis’ left hand clutching a cut along his belly. It wasn’t half as bad as hers, but it was a start- blood from the man who hadn’t bled in a decade.
“You deserve my respect after all, Madam,” he allowed, still smiling. She lunged again, faltering when the move drew fire across her shoulders; his rapier flicked out to catch hers, without his speech slowing or his expression changing.
“I confess some regret that you wish to kill me. You’re much better than your brother was.” Camille used the words to give her strength, to ignore the pain. She lashed out quickly, aiming for the Marquis’ wrists, his legs, then his shoulder, a lunge to the chest; he stopped all of them, not without difficulty, but she hadn’t touched him. At the going rate, she’d be too faint from blood loss to defend herself within a minute or two. With that in mind, she decided to try something very stupid. She let her point drift, drew in a breath. She forced herself to block the Marquis’ poke at her right foot at the last possible instant, with barely enough strength push the strike away. He went for the kill, just as he always did. Camille shifted right, the edge of the Marquis’ sword drawing another cut across her collarbone, just as her own swept into his heart.
He jerked, frowned at her, and sputtered, “Damn amateurs.” Then he fell, his sword planting itself in a stump next his body. Camille watched him for a second, gave him a good kick to make sure he wasn’t faking, then a poke through the skull for good measure.
“Blasted snobs,” she returned, turning heavily back towards the manor.
You get the idea, yes? The more you build up this rival character, the more intense the fight will have to be, to the point where soon you can’t meet your readers’ expectations and the whole thing will just be a let down. It’s one thing to have the grunt characters die rapidly, pathetically and without putting a scratch on the hero. Whether your villains achieve some notable scratching or not, they’re your villains because they’re hard to deal with. They’re survivors, hardened blackguards, and killing them should never be an easy thing.
Because there’s nothing worse than talking a bunch of crap right before having your face rubbed in it.