Get All Boxers:
Put in the following for your Boxer's Name: MIKE TYSON

Get All Arenas:
Put in the following for your Boxer's Name: LENNOX LEWIS

Max Out Stats:
Put in the following for your Boxer's Name: JOE FRAZIER

Get Big Heads:
Put in the following for your Boxer's Name: DAVID TUA

Be Invincible:
Put in the following for your Boxer's Name: MUHAMMAD ALI

Heavy Boxers:
Put in the following for your Boxer's Name: RICH OBERDICK

Boxers Pass Gas:
Put in the following for your Boxer's Name: ADAM GARDNER

Stupid Boxers:
Put in the following for your Boxer's Name: JESSICA MELTON

Get Scrapyard Arena:
Win all the survival modes to unlock this

Be Practice Robot:
Finish training mode to unlock the robot

Get Odive Unlocked:
Win the 3rd tournament to unlock Odive Character

Unlock InterContinental Tournament:
Win all 5 default tournaments

New Outfits:
Finish Life mode with a character to get a new outfit for that character

Get some Energy back:
Press X repeatedly after being knocked down