This trophy isn't as hard as the "Unchallenged Champion" trophy, but to some this will still be difficult.
You are required to complete a characters entire movelist in command training.
Menu > Training > Command Training.
I personally found Brad Wong to be the easiest character to do, but he has fair amount of moves.
If you want a quick & fairly easy character, Rachel is a good choice. She has less than 70 moves in total.