I second badna0's suggestion. I seem to recall many of the cards having a section listing what skills required the card for mastery. That no longer seems to be there, now. Some examples would be Dancer Michel Cards (x3) used for mastering Shard of the Glacier and Slithis Card (x5) for Shield of Heaven.
Trickster Online (Fantasia)
Cheongryong : Wizard | Baekyong : Priest-to-be | Haetae : Scientist-to-be