Summary: According to the PS3 Trophies site, you need to win total of 100 games either online or offline.

 

Countless duels” is a total of 100 wins, either in online play vs other players or offline play vs the computer. The easiest way to get this is to play vs computer, and pick starter deck for him. To do all the other trophies you get 37 wins or more, so i would advise to get theese trophies first then go for this one.

 

Sourcehttp://www.ps3trophies.co.uk/trophydetails.php?Gameid=102&Trophyid=38