The Spartan Way: Except for the lack of Drill Sergeant Nasty (which see), the Imperial Legions really do train like this – for the principal reason that, well, they are heavily dependent, given the Empire’s population demographics, on quality over quantity, so they want the most absurdly close to impossible quality that they can possibly achieve.
The casualty and even the dropout rate is refreshingly low these days (due to modern techniques of assessing psychological breaking points and that modern noetic technology lets you resurrect the dead – but still, if you die more than twice, or twice on the same test, you fail. Although you do get to live again, just not to continue the training program beyond that point; the Legions also need auxiliaries), and also due to the mental, genetic and nanotech upgrading that comes as part of the package (see: Super Soldier), but it’s still really, really harsh.
And would, yes, violate most of the Laws and Customs of War were you to do it to the enemy.
And, yes, that same modern noetic technology does let you conduct occasional live-fire exercises and do ANBC training with real radiation, nanoburn, and nerve gas. (Although due to the expense of replacing bodies and equipment, Valhalla-style live-fire exercises are reserved for virtuality training.)
But then, if after the training the actual war feels like a picnic in the woods, then it’s pretty much done its job right, right?
Also, importantly, contrast Cultured Warrior, since the Empire doesn’t want to Sacrifice Basic Skill For Awesome Training – on the modern battlefield, the inflexible die fast – nor do they want the people responsible for protecting society to end up with the distorted perspective that comes from a life of fighting and only fighting.