Is the TV show about to go into this fight? Let me spoil it for you:

from this page: awoiaf.westeros.org/index.php/Gregor_Clegane

Gregor fights in his usual style – with heavy armor, his thick oaken shield and his greatsword in one hand – but Oberyn outfights him with speed and a poisoned spear. Though most of Oberyn’s attacks are deflected by Gregor’s plate, Oberyn cleverly attacks the joints in the armor, and eventually wounds Clegane after the Mountain tires from chasing Oberyn. After being poisoned and crippled by numerous wounds, Clegane manages to grab hold of Oberyn and crush him to death. Before killing Oberyn, however, Gregor roars that he killed Elia and Aegon for all to hear.[3]

Despite Gregor’s victory, Oberyn achieves his revenge for his murdered sister as he had coated his spear with a deadly poison which resulted in a long, agonizing death by mortification for Ser Gregor.