The guide that TheFirstWonder wrote is pretty accurate for the most part. I've just unlocked this achievement back to back on two separate profiles and I think I have finally figured it out.
-Defeat her quickly, as the other guide says. (within a couple minutes)
-Kill her before she transforms into the worm
-You can take damage, but do not heal
-Finishing her with the fire sword is perfectly fine. I did it both times and the achievement -unlocked with no problem. So use the fire without fear.
-Now for the thing that I did that doesn't seem to be mentioned by anybody and it would apply to "with grace." I parried as much as possible instead of always dodging. Every time she used her jump shot, I parried it. I also parried her melee attacks in the first stage of the fight, but dodged them in the second.
Now of course I can't say 100% which of these things are and are not requirements, but any other combination of tactics wouldn't get it to pop. As stated, playing like this got it to unlock for me twice, on the first attempt on the second account, no less.
Edit: True Veteran in the comments said he/she got it to unlock following these steps except they didn't parry. So that may not be needed. If you're comfortable with your timing, go ahead and parry as it opens her up for attacks. If not, try just dodging.