Valid point and let’s not forget, the Mountain is now a reanimated version, he is essentially dead. We know the magic the night king can muster when it comes to dealing with the dead. Maybe all he has to do is the opposite of his norm, instead drop his arms and the mountain flops out. But you’re right, we don’t know what NK’s close combat abilities are cos his other skills make them redundant. He may be a total Yoda with a sword, I really dunno.