Type            Defeats         Weakness
Aqua            Pyro            Thunder
Thunder         Aqua            Earth
Pyro            Forest          Aqua
Earth           Thunder         Wind
Light           Fiend           Shadow
Dreams          Shadow          Fiend
Shadow          Light           Dreams
Fiend           Dreams+Shadow   Light
Wind            Earth           Forest
Forest          Wind            Pyro
Divine          None            None
The above chart I made is pretty self explanatory. The words under the "Type" section is
what the types there are in this game. The "Defeats" section shows what types will
automatically be defeated by the type to the left of it. Finally, the "Weakness" section
shows what Types will automatically kill the far left type of it in the "Type" section"
Use these advantages when dueling.
I'll list some Duelists and what Type of Monsters they usually use....
Rex Raptor ~ Ground
Bonz ~ Fiend
Weevil Underwood ~ Forest
Mako Tsunami ~ Aqua
Ghouls ~ Fiend