I'm looking for novels with some insane training plans and it is this way not just in the beggining, but in the later chapters as well. Examples: Running Away From The Hero The Wrong way to use Healing Magic Everyone Else is a Returnee (Sorry for the poor English , still a beginner)
Your not going to find a lot of those in the kr jp style. Since the main Korean novel website is dragging their feet on reposting their novels (wuxiaworld) and jp novels suffer from chronic beta syndrome.
you can try Praise the Orc!, though it's not hellish training but he fights every battle with his life on line....
I recall Possessing Nothing had a section in/near the beginning which the training was really harsh. I don't know if there is anymore later on though
If i remembered right, most of the novels by fang xiang will have the MC doing insane training regiments throughout the story.
Unless training involves each individual atom being ripped apart and merged together again and the process is repeated several hundred times in a single day continuously nonstop for several years ,that's only intense training not hellish where the trainee literally needs to be to angry to die .