The Mountain from ASOIAF Vs 6'2 tall, 240lb professional MMA fighter

The mountain is close enough to 8 feet tall that you might as well say 8. He's weighing in at about to 450lbs. He's strong enough to walk around all day wearing plate that would crush a normal man. He can casually swing a great sword with 1 hand for extended periods of time the way normal men wield daggers.

Lets say he gets in the ring with your average size fighter, 6'2 tall and about 240lbs. The guy is one of the top MMA fighters and practiced in many disciplines, been training for 15+ years, won many titles. Gregor only has the training he has in the books, mostly sword fighting and occasionally brawling.

I know; "People underestimate how dangerous actual MMA fighters are" but could one of them even DO anything to a guy like Gregor? There's another saying "Weight classes exist for a reason." Gregor is waaay out of his league here as far as fight experience and technical skills, but would it matter?

If at any time Gregor manages to get on top, or land a glancing blow, it's fucking over as far as I can tell.

Would the fighter stand any sort of chance, or am I also underestimating him? I'm just wondering like how useful a rear naked choke would be, when your opponent can just reach up and snap both of your wrists/forearms like toothpicks, or how useful a mounted position is when the guy can just stand up with you hanging off of him.

I don't think the MMA guy could win via striking. I know leg kicks are devastating, but you'd basically be kicking an iron pole with all the hard muscle on Gregor's legs.

The only way I personally can see a win for the MMA guy is to basically Oberyn him. Dance around out of his reach for a few rounds until he gets tired, and then get Gregor in some kinda complex move where he simply can't power his way out of, and I'm not sure how many of those there are.

Dude was exhausted, half way to dead from being literally pinned to the earth with a spear though his belly; and he still just grabbed Oberyn and obliterated him. (Toughness feat)

He's gonna be 3-5X stronger than his opponent, and surely at that point any kind of hold or submission is going to be nearly impossible if Gregor can just grab whatever limb the guy is using and use his weight and strength to reverse or break the hold. Am I wrong?

I do understand these techniques were designed to fight stronger opponents, but Gregor is borderline superhuman. If he grabs you, he can break bones without much effort on his part.

Am I way the fuck off? This almost feels like a "spite match" to me. Does the fighter have a better chance than I think? How many times outta 10 do you give the win to the actual fighter?

(I'm also aware that the guy who played Gregor on the show is doing some kinda fight training now, and he ain't much; but show Gregor and book Gregor are 2 very different things. Book Gregor is much larger and probably at least twice as strong as Hafthor, or at least lets assume so for the prompt. Also aware of some David VS Goliath situations where tiny dudes occasionally out preform ppl way stronger and heavier; the name "mighty mouse" comes to mind, but I don't think there's ever been a fighter like Gregor who could probably just grab the cage and tear it down with his hands.)

Last week my little brother snuck up on me and got a pretty decent rear choke set in (no fight experience and he's pretty weak, but he caught me good and tight). Still, I simply grabbed his wrist, pried it down off my neck and then pitched forward and flipped him over me onto his ass. I'm no Gregor by any means but I worked hard manual labor for 15 years. If it had been someone of his strength and build, or even a little larger he woulda choked them out, but he couldn't even keep the hold on me. The gap in strength between Gregor and the fighter would be waaay larger than between me and my brother. So that's where the inspiration for this comes from.

I just don't see any amount of technique overcoming a significant strength gap to the point one party is so strong they can just resist and succeed against half these moves, or whatever. Am I wrong?