The description is somewhat misleading. I hit her before she said the (in)famous line, and got the achievement.
However, if you actually die, you don't get the achievement; you must defeat her. Of course, you have infinite lives, so this should be easy. Also, unmissable storyline achievement.
If you want more advice, or have some for me, please post a comment.
Edit: I've heard this is bugged, if anyone knows how to defeat/avoid the bug, please post an explanation.