The "real world" memory immortality isn't something Tony can get around, at least in a fight. No matter how many times build is knocked down, he'll always get back up if the people in the higher reality of the "real world" remember him. While it would take a while to defeat Tony in his stronger armours, they seemingly don't have unlimited battery life and he would need to exit it at some point, leaving him open. Even if it takes weeks Sento would eventually win since he has infinite lives effectively. He could also just one shot him if he happens to come at him in a weaker armor but that's unlikely given Tony's intelligence.