1. If you're playing for the first time, pick Yugi.(He seems easier in my opinion.
2. When you play for the first time, use all your marshalls.
3. Buy potions from traders so you can use them when needed.
4. If you beat the game once, only bring your best marshalls into battle. Good monsters cost money you know!
5. Make sure the strongest monster on your team holds the potions.
Now I'm gonna do strategy for each character.
YUGI
1. Use Yugi a lot. He has some decent monsters. When you have all the marshalls: Use Tea and Mai. Tristan can be helpful too.
KAIBA
1. Kaiba is good in battles. Use him.
When you have all the marshalls: Use Kaiba. I don't really know who else is good. I swapped the monsters. Pegasus is good, I suppose. Also, Ishizu has good monsters but, they cost a lot to use!
SECRETS
1. Beat Yugi's game! You get "Soul of Obelisk."
2. Beat Kaiba's game! You get...Slipher(Osiris). I forgot what they call it in the game.
3. Beat Yugi and Kaiba's stories! You get to be Joey now! Joey's story is harder. Like every monster you face is level 50.(If I remember correctly)
That's all I know. I haven't beaten Joey's story yet. I got bored. It's not hard though. Not with my Level 90 beasts!
"Mewtwo Battle" from Pokemon Stadium.