Make sure you take all [Crossroads] on the right hand side (EXCEPT "SIMPLIFIED" QUESTLINE), once you progress to Cron Castle you have TWO opportunities per character to get this knowledge. First remove knowledge at Calpheon, the quest reward from [Balenos] To the End gives the knowledge (but you can't have it already).

The second opportunity comes from actually killing the shadow knight shortly after. I ended up getting S from the actual shadow knight after B from the quest on my third character through.

Make sure to remember to pop +% knowledge and higher knowledge chance buffs, good luck!