Most non-story achievements you can get only in one section when you play as a different character. Don’t worry about it too much, there is chapter select after the end.
For this one, when you play as Audrey (girl with a sword), hold your sword with
towards an enemy attack. You don’t need to time it or anything. Repeat 20 times.
If you are short of 20, you can still do it during the final boss fight. In the third stage of the fight, target falling stars with
towards Audrey and she would reflect them herself which counts towards this achievement. You can sit in that stage for as long as you need to, without winning or losing the fight, to get all reflections you need.