Got an A rank on Castle of Varrvazzarr. Just ignore main battle in a circle on the map. Go down the tunnel. All the enemies chase you, get to the end, pick up orb doubler and now combo with Y button and using the attack choker at -50%. Also unequip any attack boosting accessories.
I got a combo of 6,000 which was overkill. 2,000 would probably be enough, maybe even 3,000.
Now why has none gone for an S rank? Shame no one on the whole internet has posted their stats if they have.