To unlock death circuit You'll ahve to win 10 endurance chalenges and at the cirtuits you will notice a small circuit mark will be in the middle of the circuit selection list.
During the first match in the weaner vile cup punch him until he gets dizzy then hit both buttons at the same time and ... WATCH!
These are secret unlockables that can be unlocked by the other cheats.
exhibition 2 - beat death circuit
dragon rage - beat death circuit then beat exhibition 2 dragon rage will make ko kid invincible for the entire round and alot more stronger but it can only be used once
hells endurance mode - beat king pit in exhibition 2 12 times to unlock full endurance mode including death circuit
viper x - complete the whole game 100% then go to option then tap final boss to fight viper x
free roid rage - beat viper x
new acheivement list - beat hells endurance mode 25 times
viper x chalenge - beat viper x 5 times to unlock viper x chalenge mode
demo of super ko boxing 3 - beat viper x,s chalenge then go to option and tap demo of super ko boxing 3 to see it