USING RARE AND LEGENDARY POKEMON EG DEOXYS
When you are at the main menu go to "vs mode",then choose the option on the left, not "group battle".
Then I think you choose anything goes, then choose "ULTIMATE" battle. When the Pokemon come up you should have very rare ones on your team such as DEOXYS. You can also use JIRACHI, MEW, MEWTWO etc.
If you don't like the Pokemon the computer has chose for you, just press B then press A to go back into it. You should have different pokemon
We have more cheats and tips for this game here, Pokemon XD: Gale of Darkness cheats
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