When you reach a save point and there is a vase with an extra sand slot, destroy it to get the sand. Save the game, then fall off a ledge or get killed. Load your saved game and destroy the vase again. This will allow you to get unlimited sand.
You can kill ninjas easily by using combo attacks and dual wielding a dagger-like weapon while they are not blocking (after you hit them once). Quickly press Triangle, Circle(2) when they are not blocking for a one-hit kill.
The women are hard to defeat because they always block and you cannot jump over them. To easily kill them, run towards a wall and press Square. You will launch off the wall towards the nearest enemy and hit them. This is unblockable, but not undodgeable. This will also knock them down so you can stab them while they are vulnerable and use a combo on them when they get up.