In Code Geass: Mao of the Deliverance, Mao, frustrated by losing C.C. in a terrorist attack, brutally mutilates a corpse with the handle of a pistol (he already used up all the bullets!) while shouting random obscenities until he is physically exhausted.
A Dark Knight over Sin City has Killer Croc and Marv exchanging them with one another. Croc starts off by beating him to a pulp before throwing him out a window. Once Marv survives and does a little prep work he comes back, runs over Croc with a car, smashes him over the head with a fire extinguisher, blinds him with the contents, and then gets ready to send a bullet through his head at point-blank range. Luckily for Croc, Batman stops Marv before killing him.
Pony Space has Rarity deliver one of these to a diamond dog guard, beating him with a table while giving him an And This Is for... speech. She accidentally kills him.
Similar to above, Rarity delivers one to CT in I Against I, Me Against You. Considering the abuse she'd suffered as his captive earlier, it was a long time coming.
In The Wizard in the Shadows, the Witch-King tries to distract Harry by poisoning Ginny. It backfired. The ensuing beatdown vaporized a Mûmak that got caught by a stray lightning bolt. While the Witch-King does get up afterwards, he's so weakened that Theoden has him on the ropes.
Emrys delivers one to Wormtongue, and very nearly kills him. This is a malnourished 15/16 year old boy against a full grown, well fed man. Wormtongue has his face smashed in and Emrys is in the process of carefully drowning him when he gets called off.
In Mare of Steel Rainbow Dash takes a bad whack to the Berserk Button when Brainiac seemingly kills her adoptive mother, Firefly. She jumps into Unstoppable Rage and almost kills Brainiac before Thunderlane manages to talk her down.
The fights between Diana and Ares in Princess of Themyscria are absolutely brutal, usually ending with Diana beaten savagely by the much stronger Ares.
Goggles And The Tears: On Comstock's airship en route to Monument Island, while the Vox Populi are fending off thousands of merciless flying androids, and Elizabeth is knocked out from overusing Tears for backup, Booker DeWitt throws down all of his weapons in rage, and fights Android!SHODAN with his bare hands, to the point that he manages to smash out one of her optics.
Poor Chet Kelly is on the recieving end of one in "A is Also for Alone". It's so bad that he needs surgery and spends two days in a drug induced coma. The bruising and swelling are so severe that not only his crewmates but even his mom has trouble recognizing him when they see him. His mom stays a few minutes, then leaves to break down crying.
Anything or Nothing involves Pete giving one to PJ after PJ decides to fight back in a regular beating. He likely would have died had Max not come by and distracted Pete, but manages to survive and make it over to Max's house where he is taken to the hospital.
The inciting incident of Project Riribirth is Riri receiving an absolutely devastating one from an enemy that was much stronger than her previous opponents. By the end of that fight, both of Riri's arms as well as her spine are broken, and she's confined to a wheelchair for years afterwards.
Chapter 34 of Wisdom and Courage depicts Link on the receiving end of a particularly savage one at the hands of Veran, who dishes it out partially in an attempt to break his spirit and partially just because she wants to hear him scream. After knocking the Master Sword out of his hands, Veran first activates the curse she earlier cast on Link to incapacitate him with pain, then slices him to ribbons with a sword, doing such things as stabbing him in the leg so deep that the blade actually hits his thigh bone, and finally going so far as to cut out his right eye with a dagger. She's only stopped from going any further than that by Zelda shooting her In the Back with a Light Arrow, and by that point, Link is puking blood and literally beggingVeran to stop.
In Syngenesophobia, Lincoln receiving one of these from his sisters after accidently angering them is what kicks off the plot. It results in a broken nose, broken arm, black eyes, fractured ribs, a bruised pelvis, some kicks in the nuts, and some teeth getting knocked out. And those are just the physical injuries as Lincoln also develops a crippling fear for his sisters due to the severity of the ordeal.
In Shattered Innocence, Lynn is on the receiving end of two: one from Lori, which renders her nearly immobile, and another from Ronnie Anne, who forced a fork into her hand, beat her face in, and stomped on her arm until it was broken.
In Chapter 7, after being mocked by Dirk, Lynn savagely beats him when he insults her brother. It results in black eyes, missing teeth, his face getting repeatedly bludgeoned against a chair in the cafeteria, and him nearly dying.
The first one occurs when Lincoln finally stands up to his family and leaves, but not before calling Lynn and Lincoln, leading to a particularly violent scrap between the two. This culminates in Lynn getting her head bashed into the back of the couch, Lincoln taking Lynn's thumbs into his eyes, getting kicked repeatedly after a Groin Attack. Finally the two bite eachother to the point of drawing blood, with Lynn biting Lincoln's hand and Lynn's leg being bitten so hard that she won't be playing sports for a while. All this is emphasized with this description of their anger.
There are 10 stages of anger, from lowest to highest: bothered, mild irritation, annoyed, indignation, frustrated, infuriated, hostile, wrath, fury, and lastly, rage. In Lynn's case, her level was that of indignation. When Lincoln came home, his aggression was up to infuriated, but thanks to that one punch, his anger was pushed beyond the boundaries of 10. Ladies and gentlemen, welcome to level aggression 11, and enraged doesn't even begin to cover it.
The more satisfying one occurs when Lincoln witnesses Chandler and his friends throw Lana into a lake and watch her cry. Lincoln gets pissed and inflicts the following beat down on all of Chandler's gang; using a tree branch to break ones nose and inflict a Groin Attack on another; breaks that branch while hitting Kevin, using what's left to beat him down. Lincoln proceeds to suplex Luke before knocking him out and when it comes down to Chandler, Lincoln tackles him to the ground and gives him a beatdown that ends with Chandler getting swollen cheeks, missing teeth, a broken nose and a black eye.
Lincoln: What is my name Chandler?... WHAT IS MY NAME?! AND CALL ME 'LARRY' ONE MORE TIME! I FUCKING DARE YOU! Chandler: L-L-...Lincoln!... Lincoln: You can pick on me all you want...but if you, or your friends, go anywhere near my sisters or mess with my friends again...you better pray, that I don't find you..."
In The Day Everything Changed, the main antagonist, Sakura Takahashi, inflicts one of these onto Kagami, beating her to a bloody pulp with a metal pipe and slashing her twice with a pocket knife. Luckily, this presses Konata's Berserk Button and Sakura quickly gets what's coming to her.
In Dark World, this is basically Angry Pie's entire fighting style. Of note is when she beats down Twilight and rips her Element of Chaos out.
She's on the receiving end when Twilight manages to get a new Element of Magic to replace the old one and becomes a Proto Nightmare. The beating is so brutal and violent, when Twilight is finally calmed down, she wonders how many times over she just killed Angry Pie. Angry Pie is left catatonic.
Earlier, Dark World!Derpy Hooves beats the Valeyardto death after his regeneration template is destroyed and he's pushed her past the Rage Breaking Point. By the end, he's reduced to feebly begging for mercy. She doesn't have any for him and implodes his rib-cage.
During the Finale Arc, a post Heel–Face Turn Nightmare Bannerette (the one who served Eclipse) delivers one to Abandon at the end of their fight. Said beatdown involved literally stomping his head into the ground until he dies. Given Abandon had tried to corrupt three versions of Rarity into being his vessel for entering reality and Bannerette is a god version of Spike, it's understandable.
Used in a very realistic but horrifying way in the Kalash93 story, You Obey. What makes it so effective is the absolutely brutally honest depiction of what real interrogation and torture are like for both the interrogator and the detainee.
Twilight gives one to Cadence here.It lasts for several hours.
The Pieces Lie Where They Fell: Night Blade delivers one to Page when he mistakes her true form for a Changeling impostor. He deeply regrets it when he realizes the truth.
In Once More with Feeling, Shinji's return match against Sachiel. After ultimately deciding against waiting for Unit-01 to go berserk on its own, he takes advantage of his previous synch ratio and combat experience to literaly tear the Angel down, leaving it almost no chance to retaliate. He even manages to contain the blast caused by its self-destruction with his AT-Field, and acts confused after the fight to pass his combat prowess off as Unit-01's awakening.
In A Pikachu in Love, an Ursaring delivers a pretty nasty one to Pikachu due to him being too exhausted to properly fight back, as well as being forced into cramped corners where his main attribute (speed) is of no use. Poor Pikachu nearly gets beaten to an inch of his life until Pichi finally got up the courage to save him.
This happens to several characters in Natural Liberated. N delivers them to Ash, Rourke, and Clemont. He does so to Clemont for information.
In Old West, Bad Bill and his gang give Teddy a beating as an incentive for his mother Grace to sell Dufayel her lands. Unfortunately for the thugs, they only manage to make Grace absolutely fierce, and she proceeds to give them all a bloody beating, much to Rattlesnake Jake's astonishment.
Sonic: Evil Reborn Zero has a few, but the most brutal one takes place during Chapter 6, Episode 2 "An Easy Detail" where a Sonic Android not only thrashes Shadow nearly to the point of death but also uses its similarity to Sonic to convince him that Sonic's friends were only pretending to like him.
In chapter seven of Bait and Switch, Eleya, who was previously nearly killed by an Orion matron during a boarding action against the frigate she was serving on, disarms, disables, and beats to death another Orion matron. She had to be physically tackled off the dead Orion and pinned down by her first officer.
Offscreen in "Remembrance of the Fallen", Tia overheard an idiot armchair-quarterbacking civilian say that her brother's death defending a convoy from Orion pirates was a Stupid Sacrifice and that he could've done it without dying, and the only reason the guy got off with just a broken arm was because that was when SFPD showed up.
Leia unleashes with one against a woman who tries to kill Han for killing her father in Worst Vacation Ever. It was actually Vader who did it, but the woman doesn't buy it, shoots Han in the arm, and then tries to shoot to kill while he's on the ground. Leia promply snarls and lunges at her, beating her down quite severely. Afterward, she says "No one hurts someone I love".
Han endures one in The Way It Should Be to protect Leia. They've both been captured, and although orders are to keep Leia alive until Vader arrives, Han isn't quite as important. The guards decide to have some fun by dragging him into a cell with a big guy. They tell him that if he doesn't take what the guy dishes out to him, if he makes any move to fight back or defend himself, they'll let the guard who threatened Leia earlier rape her. Han and Leia aren't a commited couple yet, but he does care for her deeply, and he silently endures the severe beating for her sake and finally they have enough and throw him back in with Leia.
Han also endures one in The Princess And The Pirate. He and Luke and Leia are mostly bait to draw in Wedge Antilles, and Han is the first one drug off, hung in chains and severely beaten. The imperials threaten to go to Luke and Leia next but the gang doesn't sit around and wait for it; they escape. Or rather Luke does, he has to come back and rescue the other two later on.
Hellsister Trilogy: During the first arc's final battle, Satan Girl lays a brutal beat-down onto Supergirl.
And with that, she cracked a blow to Kara's face that broke her nose. The static-shriek of pain in the telepathic plug was a sonata to Satan Girl. Smiling for the first time in minutes, she followed with another wallop, and another, and another. Four unanswered blows. Kara tried to resist, but her defense was fading. Satan Girl pounded her. Over and over again. [...] <Kara?> Satan Girl hesitated. She had been laying into Supergirl like a butcher on a steer. But she had been so busy in her fantasy that she had taken no notice of what she had really done. Supergirl was not moving. Satan Girl smashed her a few more times, just to make sure. Then she listened for a heartbeat. Nothing.
In Things We Don't Tell Humans, Prowl goes into Heroic Safe Mode and reduces Juryrig to a quivering heap after Juryrig rips off one of Prowl's sensitive "doorwings" in a sparring match (after having been explicitly told by their teacher that they were off-limits, presumably for that reason).
Skin has a few of these, most of them during the Virtual World arc thanks to the sudden use of Who Wants to Live Forever?. Even worse for the one exception to the temporary rule, a battle between Yami Malik and Ryuzaki using the Millennium Rod against a duel disk in 'sword combat'; while the 'bad guy' lost in a way, the 'winner' wound up comatose for a month to contrast the single hour of sleep it took for the other to recover.