Keep killing those Spike Dragons until you get at least one Kusanagi.Spike Dragon gives Kusanagi (Best for Rodi/Akane. Even better than Musamune & Muramasa. Power attack granted by Kusanagi is Critical Strike. Kills one enemy instantly. 50%-75% chance of executing a critical strike. Unbelievable. Good luck getting one)
Suggestion: Get two Kusanagi and put both Rodi and Akane in the active party.