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.