Attack them using combo attacks and when you have struck them and they are not blocking dual wield using a dagger-like object and they are then easily defeated. To perform a one-hit kill quickly press Triangle, Circle, Circle when they are not blocking.

During game play when you reach a save point there will be a vase with a extra sand slot, destroy it to get the sand and save the game. Then get killed and load your saved game and destroy the vase again, repeating this will allow you to have unlimited sand.