I have almost all of the challenges completed in the game but two of them have been a bitch so far and they are the collect 500 and 600 trophies. I got 400+ super fast and easy but now after 6 trophy rushes, 14 classic playthroughs I didn't obtain a single new trophy =\ they were just repeats. I wish you could sell your duplicates. The best way I have found at this point of the game is to spend my gold at the shop. However, I didn't realize at first that if the trophy isn't blacked out then you are buying one you already have >< whoops I bet I spent a shit ton of gold before I noticed that =P. I really want to 100% this game and with only 6 challenges left I might fall short as beating it on classic 9.0 seems impossible for me =\ I can get all the way to Mastercore without a problem but fuck he is hard, best I've done is 7.5.