Play through Story Mode until the end. Once you complete the final battle (Chapter 14), you will be shown an Epilogue video. Once the video starts, the trophy will pop so you may skip it if you wish to speed things along.
If you are struggling with any battle in the Story Mode, try using Easy Mode which has a built-in Story Assist system making combos and special moves easier to performing than usual. Also, do note that some of the final battles will end with the CPU attempting a Rage Art, if you notice yourself gaining Rage Mode also, attempt to perform your Rage Art before the opponent connects with theirs.