Eternal Idol
Lono, "The Dog"
I'm not so sure about that. Since we're using real boxers, we have to take a realistic look at Robin (Tim Drake). He's strong for a teenager and is pretty fast and skilled, but it'll only take him so far. He might be a great fighter, but in a strictly boxing match, a good chunk of his skills are made obsolete. He's in the league of the super middleweights, or at least would be able to keep up with them, but skills and experience will only take him so far until he is outclassed physically and possibly outskilled by the some of the heavyweights.
I'll give him some leniency and say it's possible to get past Marciano (considering heavyweights have generally gotten bigger over the century) but no further.