Kick him in the balls, hard.
Lots of people have said this. It's just not that easy. Without the physical training, there's no way you're going to be able to move your leg fast enough to have any resonable chance of landing it against somebody who knows what they're doing.
True story: light sparring match with a newbie karate student. Orange belt, so probably 3-6 months of training. It was supposed to be practice for him, so I let him lead the fight. Basically I just stood there and let him do whatever he wanted, avoiding attacks and occasionally tossing out a jab.
So he decides he's going to kick me. I see that he's about to kick me. Sooner or later he'll get around to actually kicking me. Ok, he's now in the process of kicking me. Damn...this guy is
slow. So before his leg finished extending I picked up my lead leg, placed my foot on his knee, and slammed his leg into the ground, knocking him totally off balance in the process.
All of this took place in the time it took him to try to get off a front kick. Probably less than a second. This was a person with
months of training time, but he simply wasn't fast enough...he simply could not move his leg fast enough to land the kick. Granted, I had several years experience on the guy, but anyone with any significant amount of training time is going to fairly easily be able to block, dodge, shift their weight so their thighs absorb the attack, or simply punch you in the face before your leg gets anywhere near them.
they'll outlast you fitness-wise
This right here is the deal breaker. You can theorycraft all you want, read and talk to people online, learn "techniques" from friends, but your body still has to be able to perform.
When I wrote my first post I was assuming inexperienced fighter against inexperienced fighter. Random flailing of arms, and nobody hitting hard enough to do any real damage. After 10 seconds it goes to the ground, people get tired, both combatants walk away bruised and sore but uninjured.
In a scenario like that, yes it's realistic to learn enough in a few hours to have a very good chance of dominating the fight. But against people who've actually had significant training, it's going to be really hard to hurt them. Most people with no training simply don't hit very hard. I recall a friendly match I once had with a guy with no training at all, I got careless and he landed a punch to my face. It didn't hurt. It kind of surprised me how much it didn't hurt, so I let him hit me again. It didn't hurt. So I dropped my guard completely and he stood there for a good five seconds punching me over and over in the face. I would describe it as uncomfortable. Eventually my nose started to bleed and I put a stop to it.
Most people don't hit very hard until they've learned how to punch. Some people
hurt themselves the first time they punch anything solid. If you don't believe me, go punch some drywall. If you've had no training, if your knuckles aren't conditioned to delivering blows, odds are good the drywall will hurt you far more than you hurt it. With training you can punch straight through it. Without training, if you can even convince yourself to really try, you'll probably lose a bunch of flesh off your knuckles and barely dent the wall. That awareness, that subconscious knowing that you can get hurt tends to hold people back from punching with even a fraction of their strength.
And when someone with a lack of knowledge of how to punch and fear of being injured tries to apply their fists on somebody who's spent a couple years getting kicked in the face by experts and being smashed into the floor during grappling excercises...the end result is that they don't get hurt much.
You've all probably seen boxing matches. Those guys take a lot of hits because they've practiced taking hits. They're good at it. It doesn't hurt them. And every one of those punches is probably ten times as much force as an untrained fighter could possibly manage even if they were willing to tear the flesh off their knuckles to do it.
This quote from the OP comes to mind:
Just this monday the one who practiced boxing
broked the MMA guy's nose, made him bleed from
marks on his face, and there was blood literally
all over the bathroom they fought in.
Yeah. That was probably a friendly sparring match. And they stopped not because the guy was too hurt to continue, but just because it's damned inconvenient when you're bleeding all over the place.
Even if you know
how to punch, even if someone explains to you how to stand, how to breathe, how to hold your body, what to guard, what to avoid...and even if you somehow remember it all and don't resort to wild flailing in the heat of battle...it's going to be
very difficult for your body to physically perform well enough to be more than a nuisance to somebody whose body is conditioned to shrug off ten times as much damage as you're capable of dishing out.