Roland Deschain and the rest of his original Ka-Tet from The Dark Tower series get put through training from hell by their hard-as-nails instructor, Cort. Borders on The Spartan Way as it is hinted ALL males born to the line of Eld go through this, but falls short of The Spartan Way as the Gunslingers do not represent the army of Gilead, but are in fact ambassadors, leaders and lawmen.
The Gunslingers like generals in Gilead's army of militiamen. The books made it clear that the original gunslingers were from the families of the Knights of Arthur Eld, they were never numerous to be an army in their own respect.
In order to increase their numbers as fast as possible, the Asha'man in The Wheel of Time force their trainees to use their powers constantly, for everything from common chores to extremely dangerous attacks. And that's when they're not busy training to be blademasters. This naturally incurs heavy losses to death, burnout, and insanity.
The training Aes Sedai put their novices through isn't far from this either. The learning to wield your power -part is, theoretically, strictly controlled for safety and the novices are not allowed to channel except during lessons, but nevertheless they'll have to practise secretly just to keep up. And let's not forget that the lessons and the chores the girls have to accomplish exhaust them so that, as one character puts it, "After one month you'll want to run off with the Traveling Folk." (paraphrased) Oh, and when the novice becomes Accepted, it gets worse. Also, the training Aiel warriors go through probably qualifies.
Perrin's training to master the Wolf Dream in the thirteenth book. Since he's in a hurry, he accelerates his advancement of dreamwalking skills by literally diving into random peoples' dreams and beating the crap out of their nightmares. But when he's done, he can pull off ridiculous Matrix stunts in the World of Dreams, up to and including blocking the unblockable balefire.
The prevalance of this type of training in his writing probably stems from Robert Jordan's experience with the military, boot camp, and his time at The Citadel.
In The Pendragon Adventure series, Bobby goes through this ("Warrior Hell 101" is how he puts it) during the sixth book (The Rivers of Zadaa) from Loor, her acolyte, and Alder. Though he willingly admits that he's definitely not the best fighter around, his skills improve immensely (and pay off at the end of the book).
In Amelia Atwater-Rhodes' Nyeusigrube books, the Triste witches have several methods of training. Some go slowly, cultivating power in generations. Then there is Pandora. Her training yields the strongest Tristes with the quickest results, but her method also has the highest fatality rate.
To quote a Death Korps of Krieg watchmaster (sergeant) from Dead Men Walking: "If you are not trying to kill each other, you are not trying hard enough." This was said after nearly killing a reluctant trainee, who was not from Krieg, and likely to have been conscripted instead of volunteered.
In The Dresden Files, the magical community at large seems fairly keen on the concept of pain as a motivator.
Harry Dresden himself mentions having gone through this. In Small Favor, he trains his apprentice in shieldwork by having her family throw snowballs at her. Her mother asks Harry if he went through something similar. The answer? Yes, but his mentor used baseballs.
When Morgan chides Dresden's training of Molly in Turn Coat, Dresden asks about when he was taught shields; Morgan says his mentor used stones. Worse in hindsight when you remember who his mentor is. Apparently using baseballs wasn't just a matter of Justin being evil if Luccio did it too.
In Ghost Story Lea is training Molly and has upgraded to rock-hard chunks of ice, and informs her that next time she'll be using knives. Harry objects, and Lea argues that given that people are trying to kill her with bullets and worse and she's currently incapable of blocking any of them, Harry didn't do her any favors by being nice.
In Cold Days, we find out that Mab's version of physical therapy is "once he's lucid enough to remember his own name, try and kill him once a day, a different way each day." Usually in the middle of more traditional, mortal-style physical therapy. Mab justifies all of this because she cannot abide by weakness in her servants. If Harry dies, it just shows his unworthiness of being in her court.
Also in Cold DaysSarissa, the changeling Mab assigned to be Harry's physical therapist through the training, was herself being trained for the possibility of becoming the next Winter Lady in the event of Maeve's death.
In Cold Days, Lea's reasons for such methods are revealed to prepare Molly for possibly becoming a Sidhe Queen should one of the Ladies die.
Jim Butcher also used this training in the Codex Alera series, but mostly off-screen between the fifth and sixth book. Tavi has great potential power with furies (elemental spirits) because of who his father was, but his power came in years later than most people. By the end of the fifth book he has mastered some internal magic and a little brute-force stuff, but is still way behind most people of his caliber. So starting after the end of the fifth book he gets a crash course in furycrafting from a Spirit Advisor. Most of the injuries we see are self-inflicted from his attempts at learning to fly.
Opinion is divided as to how far Guy Sajer's book The Forgotten Soldier counts as autobiography, or just as embellished reporting. But several chapters are devoted to the brutal training regime of the elite Wehrmacht infantry division Grossdeutchland. An Army elite that modelled itself on what it saw as best in the Waffen SS, training was in 36 hour blocks that left only six hours in between for sleep and personal amintainance. live ammunition and explosive were routinely used, it was ont unknkown for a percentage of recruits to die on field exercises, and men who dropped out were subjected to a punitive prison regime.
In Dune the Imperium sends its elite Sardaukar troopers to the hell world of Salusa Secundus for training — thus arguably been Training In Hell. Interestingly, Salusa used to be the capital, having been ruined in an attack, and is therefore kept inhabited purely for this. In the course of the novel, Paul figures out that the Fremen of equally hellish Arrakis are the only people who can match the Sardaukar's skill.
In truth, Paul verified it, but it was his father, Duke Leto Atreides, that first suspected and realized the nature of Arrakis and the Fremen. And even he only saw the tip of the iceberg — the Bene Gesserit were guilty of the same oversight when they sent their Missionaria Protectiva agents to seed legend into Fremen culture and wound up providing the basis for the desert people to have their own Reverend Mothers.
Extreme training is a theme for many groups in Dune's universe, with various aims. Mentats, Bene Gesserit, etc.
In Pride and Prejudice and Zombies, Elizabeth Bennett went through this kind of training in China so she could fight the zombie menace. It included things like having to do a handstand for six days.
In The Chosen by Chaim Potok, Danny Saunders is destined to be the rabbi (actually rebbe — yes, some philo-semitic goyim do know the difference, but rabbi is good enough for the momment) of a New York congregation of Russian Jews. Danny is shunned by his own father because "A rebbe must know what it is to suffer". At the end Danny passes the test and receives a "Well Done, Son!" Guy.
A Song of Ice and Fire: The training of the Faceless Men involves being temporarily blinded, then made deaf, then crippled — and learning to fight and survive with disabilities.
The training of the Unsullied is definitely this, beginning with the removal of their sexual organs, and continuing with a training that combines extreme physical stress with deliberate attempts to strip them of any personal identity. They are left without family or names, and are considered some of the finest soldiers in the world.
Deconstructed in Ender's Game: Ender Wiggin does not have it easy, especially being 6 years old or so at the beginning of the story... After enlisting him in Battle School, the military changed the rules of the school multiple times to make his life as hard as possible in order to bring out the best of his abilities. And purposefully provoked other kids into bullying him. After this training has finished, he went through another Training from Hell in Command School. The result is that he's a brilliant commander but also a emotional wreck that culminates in him sacrificing his entire fleet and blowing up a planet in an attempt to get expelled, then he finds out it was real and he just won the war.
A Mord-Sith trainee in Sword of Truth is captured at a young age, tortured mercilessly with a handheld Agony Beam, forced to watch her mother die, and finally forced to torture her father to death. It's said that the candidates for this treatment are selected based on their sweet-little-girl properties: the more gentle and caring she is to begin with, the more ruthless and sadistic she'll be after the training.
Sethra Lavode seems to put those she trains through this in the Dragaera series. In the prequel books, there are references to a number of her apprentices trying to kill her in response to the training, and she puts the future Empress through severe training to enable her to cross into the Paths of the Dead and retrieve the Orb and be able to come back alive. In the Vlad series set in the present, Vlad meets the character Telnan in the novel Dzur, who is being trained as a future Lavode (basically an expert fighter/sorcerer). Telnan cheerfully tells Vlad that he has only recently been allowed to leave the dungeons.
Bones from the Night Huntress books puts Cat through this, regularly beating her unconscious and chasing her through dark forests at breakneck speeds. It helps that he can use his vampire blood to heal her injuries.
Kate Daniels receives this from her stepfather for her entire childhood, beginning from the day she took her first step. Early training included leaving her in the woods with nothing but a knife, and forcing her to run wearing a backpack full of rocks. At the age of twelve he left her in the ghetto for a month and she was almost raped twice. At the age of thirteen he entered her in a gladitorial arena to fight in multiple consecutive death matches.
In the first Empire from the Ashes book, Colin has to undergo training from hell to master all of the radical enhancements Dahak wrought on his body.
Alex Rider. Most notably the first book where MI6 sends a 14 year old boy through SAS training. That's training which starts out with 200 adult heavily trained soldiers and only about 30 make it. Not to mention the unit he's put with don't make it easy on him.
All agents in CHERUB undergo three months of basic training at the age of 10 to 11- an experience which is explicitly designed to make anything they go through on missions later seem easy. Trainers get away with doing things to preteens that would get instructors on adult training courses punished for hazing. The job of running the training requires so much sadism, in fact, that when Mr Large breaches the code of conduct in a major way, he still can't be fired because the CHERUB bosses can't find anyone else willing to do his job.
The HIVE Series: From what information we're given, Raven seems to have spent her childhood in this kind of training, the basic rule being, try not to get killed.
The Fourth Year at Brakebills in The Magicians. Up until now, the students have been obliged to merely study the circumstances that effect how their spells are cast in books; here, they have to learn how to act on them without even thinking through a gruelling training process that begins with the Fourth Year students being transformed into geese and sent on a journey from New York to Antarctica. There, at Brakebills' southern campus, they are returned to human form and promptly put under the tutelage of Professor Mayakovsky, who renders them mute for the duration of their stay to ensure that they can't be distracted by friendly communication. In between mind-numbing bouts of magical training, they are taught how to transform into polar bears and arctic foxes and endure the embarrassment that results from combining an animal's instincts with several months without sex. And at the end of the year, there's an exam that forces the participants to walk to the South Pole — naked, with only their magic to protect them from the elements. (The last part is seemingly optional, although they're not informed of that. All but two students flat out refuse to do it, and suffer no apparent consequences.)
Margo views her training to be a Time Scout as this, though it's fairly benign studying and training. However, when she gets lost in Ancient Rome, it definitely qualifies.
Skeeter Jackson's childhood was a particular kind of hell. From a benign, uncaring limbo to an incredibly harsh and abusive love.
In Helm, Dulan de Laal subjects his son Leland to this for seven months after he wears the Helm — Leland is forced to undergo every kind of manual labor Dulan can come up with while being attacked without warning several times a day with bamboo sticks by his brothers.
In the Warrior Cats series, the training in the Dark Forest involves fighting with claws unsheathed, and all situations could very easily result in a fatal injury. Bonus points because it literally is Training from Hell — the Dark Forest is the feline version of Hell.
Labyrinths of Echo had kings following a special magic tradition which allows them to ensure the land's prosperity. Thus each crown prince is trained on an assumption that in such circumstances few people are insane enough to attempt regicide, but everyone and their dog will try to influence them. The monarch may look affable, melancholic, nice and rustic, or fatalistic — but if necessary, an unflappable badass with steel self-control and Compelling Voice emerges in an eyeblink. So the explaination why an old-time king born with Arbiter power managed to avoid accidentally killing himself or even messing up the world around too much (which the protagonist barely avoids despite several near-crippling safeguards) was simple: "he was a crown prince".
"The Short-Timers" has Parris Island under Drill Sargeant Gurheim. Multiple people suffer breakdowns during training, to the point where there was a standard way of killing yourself. The last one mentioned follows a different path.
In the Tortall Universe, training to become a knight is very difficult. Noble children at the age of ten train for four years as a page, then four years as a squire, then take an Ordeal that kills or drives some insane. Their busy, grueling schedules are remarked upon as being difficult to adjust to; as a new page Alanna at one point thinks she's too tired to quit and make the long journey home, but they do adjust to those. It was harder for Keladry of Mindelan, in Protector of the Small; as the first openly female page in centuries she faced a lot of sexism and unfairness from her fellow pages and her own training master. But Kel was enough of a Determinator to keep going through all of it.
In Robert A. Heinlein's Starship Troopers, Mobile Infantry Boot Camp is described by Rico as being extremely grueling and even dangerous at times. Out of Rico's original group of over 2,000 recruits less than 200 manage to complete their training (with a handful of recruits actually being killed from training accidents). Later in the book when Rico attends Officer Candidate School he describes it as being even harder than basic training because in addition to all the physical training and combat drills he is also required to become proficient in several academic subjects like math, science, history, military law, and strategy.
I may have given the impression that boot camp was made harder than necessary. That is not correct. It was made as hard as possible and on purpose. ... All I can say is this: The next time I have to make a combat drop, I want the men on my flanks to be graduates of Camp Currie or its Siberian equivalent. Otherwise I'll refuse to enter the capsule.
Normal soulscaper training is implied to be this in Burying the Shadow, but Gimel put extra pressure on Rayojini to the point Rayo wonders at times if she's losing her mind.
Thremius apparently sometimes puts Nip through this in The Chronicles of Magravandias. Once he hung upside-down and forced her to speak backwards to him for a week.