One man fighting intelligently with speed and a bit of luck. Or one man using brute force. Robert would have caved Gregors massive chest in with his warhammer, Bronn on a good day would simply outfight him. People like Jaime, Barristan, Arthur Dayne etc would go the same way as Bronn. Outmanoeuvre and outfight him. I think a big part of Gregors shtick was the fact that he was essentially someone who fought his way through commonfolk who probably had far less training and experience, obviously inferior armour, and just general fear of this beast of a man coming at them.
How many nobles do we know of that actually fought Gregor? Sandor, Oberyn, and Beric, to my recollection. One literally stuck a stick through Gregor and wiped the floor with him, another basically parried everything Gregor threw at him with relative ease, and the last one Gregor actually killed. Excusing cockiness causing death, that's a 2-1 trade off. Gregors big and strong, and he's skilled with a sword, but as I said an intelligent, fast fighter should win.