I think the hardest parts of getting plat on Sonic Unleashed would be the hot dog missions and getting all of the Moon/Sun medals. The Sun/Moon medals is hard because I haven't found a very good list online anywhere for their locations. The hot dog missions ones are probably some of the hardest bronze trophies I've seen in a game (I've only gotten one of the bronze hot dog trophies).
For instance, the Hot Dog mission for the last area requires you to go through a VERY LONG stage that switches back and forth between Sonic and the Werehog. The first time trial one has you finishing it in under 75 minutes, while the third time trial has you trying for under 45 minutes. The goofy part is that if you die near the end of the stage, you have to start all over again at the beginning, since checkpoints don't count in the hot dog missions.
