Therein lies the question we've all been waiting for. We have no idea. It was a cliffhanger question at the end of ADWD. But we can speculate. Since the Faith and everyone knows that Robert Strong is some massive beast and his fellow Kingsguard already suspect him of being odd (not sleeping, eating, or using the privy, or speaking) it stands to reason that he gets outed as being a Golem when he is incapable of combat. Also exposing that Cersei is in league with Black Magic and Sorcery and further fucking up the realm.
Or the Faith chooses someone they legitimately think can defeat him. Which leads us to the Cleganebowl theory. As The Hound is currently living on the Quiet Isle of monks and is possibly the only fighter in Westeros capable (along with the burning hatred to do so) of killing The Mountain.
My money is that Robert Strong is outed as not being human before he actually gets to fight. Seems a little more poetic that way.