More than stamina is the x5 durability + stamina. The gap is closer to the one-shot than it's not. And we don't really see Yu tiring out people far more durable than him by building up damage.
That's because this key of Yu barely has any fights, and it's not like we need to see it in the story for it to be a valid tactic. He just keeps punching until his enemy goes down. And I acknowledge the stamina feats but the ones that are most integral to this, that being the feats involving them taking blows, aren't really enough here.
1. The first one about the knife doesn't really mean much for stamina unless we see him suffering severe blood loss. It's good pain tolerance but that's not really useful here, the pain would have had to be so severe that it would cause shock or physical damage, and this isn't Alice Arachnid.
2. The second one is good but the point is to knock him out without needing to beat him to that extent; Unless kouji violates human biology in some way, no matter how much stamina he has, he's not going to just keep going completely fine. Being hit constantly in the head is different from any injury. His durability is the only real factor here unless we've seen his WR training and he's constantly getting soccer kicked in the head by grown men.
3. Heatstroke and getting punched are two different kinds of injury. Being able to withstand heatstroke wouldn't mean you could survive getting beat in the head over and over and remain unscathed. So, again, durability is all that really matters here.
4. This is again vague. How did she nearly lose consciousness? Was it from physical trauma, was she poisoned, what happened? There's not really much to go off from this.
5. This, again, speaks to his ability to endure physical labor while not being damaged by outside sources. But there's a huge difference between being able to operate for long periods of time outside a fight and inside a fight. The only way this works is if you assume Kouji takes basically near zero damage from Yu's attacks, which would be silly.
I also forgot to mention that the WR subjects at age 3 also had gone through physical and mental torture during an experimental phase of the WR
Which is way WAY more tame than what the 4th generation went through
Good for pain tolerance, but still vague. What did the torture entail?