While I don't have a story where it was my character, I do have a story where I was the one making that foe.

See, I was DM, and the villain I'd created was an alchemist/necromancer/sorcerer who played by the evil overlord list. First he sent waves of slowly growing power mook undead and constructs against the heroes. Then, in the labyrinth of his fortress, they got separated by the mobile maze, and each faced off against a construct specifically designed to defeat them.

Of course my adventurers complained out of character that this wasn't fair. However, when they were captured, those complaints faded as they were dragged past the 'construct lab', where they saw insanely high level constructs - like the ones sicced on them - designed for every conceivable character build imaginable, constructed over decades of prep work before starting his conquest. He had watched the adventurers fight his hordes, and then selected which construct to send against them based on that observational data.

Especially effective if you've gamed with these people previously and the constructs at the entrance specifically look made to counter characters of theirs from entirely different campaigns.

It wasn't against my character, but my Pathfinder group recently played a parody of Monty Python and the Holy Grail, in witch non of us could use the Holy Hand Grenade because it could only be used by a character with a good alignment. We have two Chaotic Neutrals and a True Neutral.
Our usual approach to battle is Witch sleeps enemy; Barbarian rages, charges, and power attacks; Druid (me) shoots enemy with bow (I took a feat for weapon proficiency); and my Animal Companion (Owl) bombards opponents with Alchemist's Fire.
Our GM made the demon rabbit thing immune to sleep but forgot to account for the Barbarian doing what he always does. He expected the rabbit to last two sessions, it lasted two rounds. He had to make the rest of the session us as we went because that was as far as he'd planned ahead.

It was a Heroes Unlimited Campaign, GM had made a guy who was nigh invulnerable, so only characters with Supernatural Strength or who beat a 18 AR could harm it, and tremendous dodge and parry stats so practically no one could lay a hand on him. My character was the only one without Supernatural Strength, being an Ancient Master, but I was the only one who consistently could hit the guy.

After about a an hour of the others missing and getting knocked around, and me hitting and avoiding getting hit, but doing no damage, I decided for gits and shiggles to used my Death Blow, which I hit on a 18-20. I rolled, got a 20, GM rolled the guys save vs Death and got a 1.

The GM had intended the guy to humiliate the group and be a recurring foe, and with one lucky Death Blow, the guy was dead. Frankly, the guy did humiliate us, when 4 members of the group were being treated like ragdolls and spending more time picking themselves up off the pavement, and the 5th can't deal a single SDC of damage to the guy no matter what he rolls, the group is pretty humiliated.

the house rules my GM runs under is that "There is an exception to every rule" and "It's possible to be the best friend someone could ask for and still be a complete psychopath" which let us fudge alignments to an extent.

Also, "you have ten fingers, use them."

I was running a Chaotic Evil Half-drow built around being bullshit hard to perma-kill, Ring of Cat's Nine, a curse to bring me back as an undead if I actually went down, a metric ton of spotcheck, high dodge, and fairly heavy armor.

The rest of the party was less gimmicky, a female Drizzt wannabe that did a solid job, a dwarf cleric, and a minotaur barbarian.

The fights almost always came down to me dodging, no-selling, or self-rezzing while the cleric drug the others corpses off to find a safe place to rez.

Cue Glastjag, a wizard entirely focused on firing a metric fuckton of spellfire every. goddamn. round.

Eventually, I went through three different rings of cat's nine, came back as a Lich and was still getting my ass handed to me because we were in a nice, wide open grandhall that he could literally fill with spells.

Then the Barbarian remembers he has a shield that let's him no-sell magic at the cost of being shit for anything that isn't magic we found back in the first dungeon in his bag of holding.

More like specifically prepared to be FOUGHT by one character.

Wind Elementals are the bane of our group's existence, despite how low-class they should be. I guess that's what happens when all you got is a Rogue, a Witch, and a Fighter at level 3. Silly Paladin, missing the first session he was needed in.

well, sort of...
Long story. it was a dozen years ago, amuzingly in a time where i did'nt knew what a RP or D20 mean (i was 12). but one summer my little brother and me ended up playing what can, when i think back about it, basically be called a RP very loosely based on pokemon gold and Emerald. He was the trainer, i was the game.
At some point in the game, the rival of his character decided after one to many loss to optimise his pokemon team specifically to beat him. (It was coherent to the personality of the rival, so it does'nt came out as cheated).
After that defeat, he made adjustement to his team to counter that specially made rival team, and somehow managed to avoid fighting the rival, so that when he eventually fought him in a plot relevant moment, there was no way for the rival to have adjusted his team to his new style.
aaah, sure bring me back.