Punch Hero: http://tinyurl.com/c3j2sjq
It's by GameEvil so there are IAP. The actual boxing mechanics are actually very good. It makes me think this would be 4D Sport Boxing if it were re-made today.
However, the leveling up is either extremely slow or expensive. You can level up without paying, but it will take a while. They have a non-stop zombie mode where you can earn gold which helps offset paying, but it's still a grind. For some basic boxing action, it's one of the better boxing games. If you are wanting to fight all the opponents, get ready to open your wallet.