Select the "Code Entry" option on the "Extras" menu to enter one of the following cheat codes.

Successfully complete Story mode. Go to the bottom of the menu to find the chapter select option.

To defeat the bosses easily, wait untill they do there special move. You'll know when this is because after they do it blue rings will emmit from them and they'll be frozen for about two seconds. When this happens, now is your time to attack. Also remember to block reguarly. When fighting Azula and Zuko at once remember to do the same thing except try to avoid one of them so you can get the other. Katara is better for fighting Azula and Zuko as her attacks do more damage on them.