When Fighting a droid, or any other, let them fire at you but when the bullet is about to hit you, press: "Block" or "L". It will bounce off your saber back to the droids.
Note this cheat will not work on the following:
Rifle Droid
Green Droid
Flame Droid
Loader Droid
or any force
Defeating the game (without defeating Darth Maul)
When you reach Darth Maul fight him for a little bit then quit the game without beating him. When you go to the Character Selection screen Darth Maul should be a couple characters down.
Kill Your Buddy
In a 2 player game or VS. mode, hold the right trigger and press A, B, X, and Y simultaneously to toggle the cheat on or off. This enables you to hurt your partner if in a 2 player game. If in VS. mode you will both be invincible till toggled off.
Level 11: Droidekas
Successfully complete the game with Plo Koon.
Level 12: Kaadu Race
Successfully complete the game with Adi Gallia.
Level 13: Gungan Round-up
The Gungan Round-up level is accessed by finding the three Gungan artifacts throughout the game. They are in the following locations:
Naboo Swamp
Near the end of the second level down in crevice.
Tatooine
On a medium size plateau up against the screen. Jump forward in a leap of faith. It is inside the hut with the Jawa in front of it.
Gungan Ruins
Near the end after the covered trap, right next to a tree and a large patch of bushes.
In the 1st level of the game, go to the conveyor belt and kill the droid riding it and then you'll see a nice colored background.
Ultimate Saber
In level 14, fight 100 opponents, including all the troop types from the game to unlock the Ultimate Saber. The Ultimate Saber enables one-hit kills on all enemy characters excluding Bosses. You can turn it on or off in menu that appears when the game is paused.
Note: You only need to get it once with one of the characters for all characters in that mode (one or two player) to be able to use it.