The Russian casually held a 200+lb man off the ground by his throat with one hand and crushed the barrel of a revolver. He's also taken more punishment than anyone in Westeros and kept going. He's also massively faster and more skilled than Gregor as well. His single fight scene has more feats than Gregor over the entire show. And based on what Russian did, he could pull off the same feats Mountain has as well.
The only fight he's in danger of losing is the one where Mountain has a sword, but he's skilled enough to take it from him and strong enough to use it. Soooo.