This achievement will not pop until the second encounter with Pirotess.
Simply crouch in the far left or right corner, spamming your highest DPS weapon (most likely a fast sword). Avoid using a bow as those will force you to stand.
She will run directly in front of you and most of her attacks will miss you. Switch between wind/fire to avoid certain attacks and to restore health, when you have a chance. Do your best to prevent both stances from dropping below 3 at the same time to maintain your high damage output.
By the time she changes to phase 2, you should have enough health and damage to finish her pretty quickly. Will o the Wisp is pretty good for this since your mana is going to be full at this point.