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.
In Ace Combat The Equestrian War: in chapter 11, Rainbow Dash is a victim of this trope, courtesy of Gilda. Then, in chapter 17, Fluttershy also suffers one brutal beating from Night Raven, even though she ultimately wins.
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.
In The Wizard in the Shadows, the Witch-King tries to distract Harry by poisoning Ginny. It backfired. The ensuing beatdown vaporized a Műmak 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.
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.
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.
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.
In A Pikachu in Love, an Ursaring delivers a pretty nasty one to Pikachu due to him being too exhasted to properly fight back, as well as being forced into cramped corders where his main attirbute (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.
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.
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.