What's the best way to drain Pixie's MP against Hitoshura?

The Red Star Strategy is pretty simple in that it uses the fact that the Recarmdra counter does not increase when the Pixie is using Dekunda or Dekaja skill.
On every round that you are fighting the Pixie and Arahabaki, have one characters cast a –nda skill, another cast a –kaja skill, and the last one to heal or pass. That will force Pixie to cast Dekunda and Arahabaki to cast Dekaja on every turn. Now the Demi-Fiend will still continue to assault you and get his lucky criticals for which the Pixie or Arahabaki will get another turn.
If Arahabaki casts Debilitate, make sure to cast Dekunda right away. Keep this up until the Pixie will not be able to cast Dekunda since she will not have the MP for it. At that point get the character which has been casting –nda skill to cast Debilitate and repeat this for four turns.
Make sure that for the rest of the fight one character will use a turn to cast the –kaja skill in order to keep Arahabaki casting Dekaja. If you forget then Arahabaki will cast Dekunda and spoil your fun.
Once you have cast four Debilitates, start the assaul on the Demi-Fiend. Have one character use Last Word on the Demi-Fiend, another heal or use Last Word on the Demi-Fiend and the last one to cast the –kaja skill.
I strongly suggest that the –nda and –kaja skills your party use to keep the Pixie and Arahabaki busy are Sukunda and Sukukaja. Good luck.