Advance your way and try to enter in the left door... You are surprised at by the Master of the Castle... Halicarnassus!
In this battle, use physical attacks because this boss has strong counter-attacks for those to use Black Magic or Summon!
The old and efficient combination Rapid Fire + Two-Handed, is very good here!
If you feel the battle hard, use Zeninage or Throw (Fuma Shuriken),...