First off, thanks for this, just got the platinum.
Here's my guide for those still trying, it's actually much easier than it sounds.
1. download the file either from here or get the 'street fighter ?' from xploder and copy to ps3 using standard method
2. Start with Challenge Mode, do the first time attack, survival and trial on normal to unlock all challenge-related trophies
3. Go online, and this is the tricky part because you need a boosting partner to get the 10 in a row trophy (Playing to win!). Play (win or lose) any ranked online match to unlock the online-related trophies
4. Go to Arcade mode, and this will be the most time-consuming part. I looked around and tried some different techniques, and the easiest one by far is the Zangief technique (as described in ps3t). All you have to do is spam the Double Lariat (press all 3 punch buttons together, assign it to a button you are comfortable with, you will be pressing it a lot!). Select Hardest difficulty, 1 round, 99 seconds. Select Zangief. Now make sure that P2 controller is active so that you can press start and interrupt the match if it is not going your way! That way you maintain your win streak and won't have to start from the beginning every time. The objective is to get LEGENDARY CHAMPION trophy, which is really easy if you follow these steps. You need to acheive the following:
*Never lose a round (if you are losing, just press start on P2, beat up P2 and restart your fight)
*Get at least 1 perfect round
*Get at least 3 super or ultra finishes
*Get at least 5 1st attacks
Now keep count on a sheet of paper so you are sure that you meet all these requirements. Go for the perfect in your first fight, like I said, you can keep restarting any fight using P2 technique. Spam that Double Lariat and after beating Seth, Gouken will appear, crush him and you will get a shiny plat!