Use the dagger to stab the enemy so that he becomes frozen. Then, hit
him once with your sword so that he floats in the air. When he lands, he
will be lying on the ground. Stab him with the dagger to take his sand.
Also when you encounter an enemy, press X to vault over him, and while
in the air press Square, then Triangle to kill him. 
Note: This works on most, but not all, enemies.