Some time after the cutscene involving him a target will appear on your map. Go there and beat him up. I found him during some memory so I just did it while I was doing the memory as well not sure if he's int he same place for everyone.
Pearl Code: 0731 1228 8254 White Code: 0862 2790 1982 SSBB: 0989-1461-9542
He's the person who has a huge red target over his head on the map that says "Target". I honestly had no idea what he was until after I had finished the game. I tried killing the target, chasing the target, and other things, but I had no idea it was Duccio, so I never tried beating him up. Anywho.. Target = Duccio. Big red symbol on the map. Give him a high five. In the face.