Predictable:
Try this: Start with Goku, and beat Dragon Universe (at least one more time) with everyone EXCEPT Broly. Start with Goku, and go completely around in the order that is presented. It has worked for a couple people,....at least that's what they said in a "thank you" e-mail that I've received.
If that doesn't work,...I honestly don't know what to tell you. As far as the difficulty setting,....try it on Average (the one before Hard). Then try it on hard....but I don't think it makes a difference. I could be wrong.
Death is but a door, Time is just a window,...I'll be Back!!!!!