Mugging. Tony was too rough and brutal for Leon to deal with, and tougher than Mercado, the biggest HW puncher Leon defeated.
Leon was a wader. He liked to just move right in, without much calculation of deterrent. Straight lines and physical strength. He might force Galento to the back foot in spots, but ultimately, he's the kind of target Galento needed to be really successful, and while not necessarily fragile, I can't call Leon durable. Once Tony adjusted to being pressured, he'd started lining up hook after hook. Then he'd knock Leon out. I don't think he'd need to show much of his rough and dirty game off in this, the bullseye is too wide open.