Veran in the The Legend of Zelda fic Wisdom and Courage. While she is indeed very powerful and dangerous, her massive arrogance, tendency to Kick the Dog as much as possible, and her constant underestimating of Link and Zelda place her in this category. In fact, during her fight with Fierce Deity Link in chapter 29, the minute she realizes that Link is more than a match for her with the Fierce Deity's power even with the Triforce and Majora on her side, she bails.
Loved and Lost: Prince Jewelius is a skilled manipulator and chessmaster who successfully usurps Equestria's throne by using the events of "A Canterlot Wedding" to destroy the reputations of the princesses, Shining Armor and the friends of Twilight Sparkle as well as corrupt the latter into turning against them. As Equestria's king, however, he quickly lets his ego get the better of him to the point that he reveals to the disgraced and ashamed heroes that he worked with Queen Chrysalis to overtake Canterlot before double-crossing her. He's arrogant whenever he believes he has the upper hand, but he's in reality a Dirty Coward who relies on his more powerful allies/pawns to do the fighting for him. He's partly inspired by Scar and even dies much the same way; he refuses to do anything about the escaped Changeling army before he's done with the heroes, and once the heroes have cornered him, the Changelings appear and take their revenge by eating him alive.
The Odds Were Never In My Favour: Dumbledore, to sociopathic degrees. He's constantly over-estimating his political capitol and squandering it on petty, self-centered matters. He completely (and regularly) ignores good advice from Flitwick and Snape warning him about mistakes he's making, or how he's misjudging the student body. He thinks that he can do four important jobs all at once even as it becomes painfully clear to the readers that he's failing at all of them. He's remained painfully oblivious towards major threats such as the Exchequer. He treats any defiance against his authority with an It's All About Me attitude. He makes deals with dangerous factions while remaining dismissive of the potential blowback and blind to any evidence of their instability or evil nature simply because their "Light" factions. By the winter break of Alexandra's third year, most of the characters agree that he's setting himself up for a hard fall, and are patiently (and often eagerly) waiting for it, while Dumbledore himself remains utterly confident of his own righteousness and invincibility.
Old West: Benjamin Hares, the estranged gopher snake husband of Grace Glossy, is a cowardly Con Man who's often in trouble due to his lack of forethought. He tries to use his still valid marriage with Grace to sell her property to Dufayel behind her back, but that fails because a representative of the bank becomes suspicious of him and fetches Grace, and the (not amoral) attorney puts the case on hold until he's provided with legal documents.
Queen of All Oni: Lung sees himself as the next Daolon Wong, and is utterly convinced of his brilliance and power. And yet, when his plan to break Jade to his will fails, he simply blunders ahead with it, even when it's clear he's killing her. This gets him killed by her vengeful bodyguards.
Rosario Vampire: Brightest Darkness Act III: Throughout chapters 30 and 31, Kano simply laughs off the girls' threats as he's blackmailing and photographing them, confident that, between the pictures he's taken and the group's current shaky situation in the academy, he's untouchable. He's proven wrong at the end of chapter 31 when Luna eats the entire back half of his head as he's about to force himself on Mizore.
Doumon from the Tamers Forever Series is a manipulative, condescending and arrogant bitch who casually dismisses Takato as a threat, treating him like a small child who needs to learn his place. Which makes it all the more satisfying when Takato calmly points out to her just how completely and utterly screwed she is.
Blaze and Indus in Team LVDR. While both are incredibly monstrous, they both fail spectacularly in their first attempt to kill the heroes.
The Very Secret Diary: Tom Riddle is certainly very dangerous and manipulative, but he's not as clever as believes himself to be. He repeatedly underestimates Ginny, and thus constructs an incredibly weak backstory. All it took was Ginny asking a few uncomfortable questions, and his ruse was undone. Even when he starts outright possessing Ginny, she still finds ways to delay his plans, to which his only response is angry, impotent threats.
The Victors Project: President Snow remains utterly confident that he's one step ahead of the rebellion's plans when he's really two steps behind, and it's his own excesses and cruelty that have driven so many victors and other influential citizens to join the masses in plotting against him. Even after the entire Nation is in Rebellion, and defeat is a certainty he still doesn't lose his cool, haughty bearing.
Uwabami swiftly proves herself to be this in the Good Neighbors series when she catches someone posing as a press intern. As she can't resist gloating to them about how the arrest will get her manager off her back, since said manager wants her to make more arrests and be more active as a Pro Hero, but she'd rather not risk getting injured during an actual fight. Not when she's got so many modeling contracts to fulfil.