SINNER: Sacrifice for Redemption

There are reports that this achievement is glitch. That means even if you get all the two requirements below, the achievement is not guaranteed to pop.
• Need to beat her fast (less than two minutes)
• Don’t let her turn into ugly worm
You can take damage. It won’t void the achievement.

Tips that might help with her fight:
- Choose her as your first boss fight
- Use your big sword (press UP/DOWN on directional button). It is slow but has long range.
- I found that walking toward her and roll forward is more effective than rolling left/right in order to dodge her attack.
- Best way to avoid her ‘raining arrows’ is by holding RT to run fast (not pressing A to roll)
- Fire imbued your sword. It will chip down her health faster (press left or right on direction button to find an icon of fire, than press RB to use it). Strongly recommend to have your sword imbued with fire at the start. [Edit: as LCDS Nito suggested however, don't finish her while your sword is imbued with fire]
- When her health is low, you need to take her down quickly while she is stunning otherwise she will turn into the worm. However, by the time she is stunned, you are more likely running out of stamina. Don't panic. Consider standing still for one or two seconds to have your stamina back to half bar, and then finish her off with (normally) two slashes.
It goes without saying that logging on to your enemy is a must in this boss fight.

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.

Just a guide to add a video to see the full fight with the achievement unlocked :

ok is simple, need to kill Lust without use healing item, when boss reach 15% health, activate the flame power with max stamina and kill first turn in a giant Worm!!