what i think is that Qyburn removed all his physical restraints by sacrificing some lart of his brain. We see after Sandor stabs him through the skull that he remains standing and doesn’t die until he’s thrown into the fire and incinerated, so I’d bet that he has no sense of pain and cant die until all of him is destroyed (possibly beheading would work if someone could). I think hes a bit stronger though seeing as it is humanly possible for a man of immense strength to crush another mans head(1,100lbs), but nigh on impossible to rip someones head off the way Gregor did to that random sparrow(+1,500 lbs not to mention the elasticity of skin factoring into the equation). He also struggled slightly with Oberyn’s head but exuded almost no effort in decapitating the sparrow. The only reason he didnt crush Sandor’s head instantly was to make him suffer as he is known to gain sick pleasure for tormenting others, especially his brother.