Mishra
Successfully complete Quest mode using a preset duelist and deck.

Get Tsabo Tavoc
Successfully complete Arcade Duel mode.

Get Volita
Successfully complete arcade mode as Kuldan.

Get Maraxus
Successfully complete arcade mode as Volita.

Hidden Level
At arena select screen press L + R, Left, Up, X, Up, Right, Y, L + R.

All Duelists
At character select screen press L + R, DOWN, UP, X, WHITE, UP, X, BLACK, UP, X, L + R.

Easy win against blue computers
This is a good hint for anyone who is surfing cheatingdome and has trouble with beating blue computers. Never kill Blue Turtles on purpose,one time my opponent had five out and couldn't do anything,computers aren't smart enough to know they should kill they're own creatures sometimes. Hope this works, I beat the game in 3 days so I know what I'm talking about.