This is awarded at the end of chapter 3 after you
You only have to get their health below 50% to complete the battle. The achievement pops after the battle.
The opponent does attack quickly with big combos, so spam your fastest attacks, maybe throw in a couple of reverse edges to quickly fill your Soul Gauge up so you can do a Critical Edge if needs be. At the time I was level 44 with a lv41 weapon that gave me back HP every 2 seconds and every CE I did