Rewarded for completing the game on legendary difficulty.
Can be done once the game has been completed on beginner difficulty.
Unfortunately it is not possible to simply change the difficulty to legendary once finished and complete the last stage. All stages have to be completed on hardest difficulty.
Difficulty can be set in the options menu.
Nevertheless, after your first playthrough on e.g. beginner difficulty you should be almost completely leveled up, meaning almost overpowered.
I would suggest equipping the jade staff (which generates Qi for every killed enemy) and rushing through each level with LT + B technique.
The final boss can be finished easily using a mixture of LT + X to keep him in the air and Y / LT + B attacks.