You need to:
- Choose the right path that leads to the red light monster
- Choose "Hesitate" when MC is about to finish off Deryl
- Choose "Destroy the sword before it's finished" when fighting against Deryl's final form
- Have at least 28 skill (choosing to overpower the red light monster will give 3 skill points and 5 power points)
In that order. Don't know why Weird World decided to make it so overly complicated to achieve a perfect score against Deryl, when the Valravn fight was handled in a much more simpler and satisfying way, but it is what it is.