Does the Mountain have amazing defensive ability that we don’t know about?
Yes.
"
Gregor wears the heaviest, thickest steel plate armor in the Seven Kingdoms. Dull grey in color and battle-scarred, his armor is so heavy that no ordinary man would be able to move, let alone fight effectively while wearing it. Below the plate he wears chainmail and boiled leather. He also wears a plate helm with only a narrow slit for vision. "
+ add to it the fact that he is using shield and was specifically praised for having great battle instincts.
Good luck trying to do any serious damage to Gregor when you don't have valyrian steel or god-tier technique.
Gregor is 100% offense, what is he going to do if Sandor actually starts fighting back?
The moment Sandor tries to focus on offense he gets torn to pieces. Gregor can allow himself to be full-offensive fighter, because again he has great advantage in both range and armor. Sandor trying to attack can create a potencial gap in his defense which could end badly for him, since the Mountain was literaly stated to be tier above him in physical strength and can cut people in half with one strike.
Gregor is 100% mindless rage. He doesn’t need to possess overwhelming skill, so he doesn’t. This is why Oberyn bodied him that severely. Sandor is much different, he is highly skilled and fights with his brain, and his brute strength is well more than enough to hold his own against Gregor.
No. Oberyn won, because he fought the way to become Mountain's kryptonite:
-using the spear instead of sword to have greater range over Gregor's longsword,
-no heavy armor to be able to dance around him and go full agility instead of taking him head on,
-using poison to weaken his opponent.
I agree that Sandor has better technique then his brother, but still his approach and skillset are different then those of Oberyn.
Sandor is literally the superior fighter here.
Nah, he has no feats to prove it. I'd pick the mountain over him although not by a huge margin.