This is a Libra of Souls achievement for reaching the end of Chapter 4 where you
The opponent is at Lv 55 and he regains health throughout the battle. I was at level 51 and I would advise having weapons which give you health back at any opportunity(I had 25% on CE hit and HP recovers every 2 seconds on a level 41 weapon.
Spam your quickest attacks but block after a few because he is quick and powerful. Get your Soul Gauge filled quickly. His biggest combo(the one with the spinning attack) is easy to spot so you have time to block it(although it has a small chance to guard crush).
Be wary because he has a habit of teleporting out of your Reversal Edge