Once you have about 98% of the game completed and are prepared to have a showdown with the evil Vizier, the best way to kill him is with counter strikes. Defend against the clone with R. Unlike his underlings, he cannot figure out a way past your sword. As soon as he strikes at you, press X to counter with your sword. You can also use your dagger, but it is much less effective due to the fact you cannot freeze him. If desired, you can hit him by jumping over and slicing with your sword, but this may not work and is slower.