You will unlock this achievement after finishing the 3 back to back boss fights of chapter 2. i was level 31 and had no trouble at all against these bosses and didn't require any healing items. The chain of boss fights begin at the end of the quest "My Target"
*** Spoiler - click to reveal ***The first fight will be against the Giant Ziczi and is fairly easy. The next fight is against an Enraged Zizci and is a little more challenging but still pretty easy. He will be able to become momentarily invincible during the fight but it doesn't last long. The third and final fight will be a solo fight against the Integrity Knight Eldrie. This fight is also pretty easy, he likes to spam one certain ability during the fight that cannot be blocked, you will need to back up and give him space when you see him preparing it. Try to avoid "finish arts" as it is very easy for him to avoid it. I suggest using "super arts" they seemed to do the most damage for me.