Use Force Speed to move faster, then attack him with as many blows as possible.

Hide behind one of the pillars in the four corners of the floor. Desan will throw his lightsaber at you, but it will cut through the rock pillars and will fall on him, killing him instantly. Note: Use the invincibility beam in the center of the room first so his saber does not hit you.

Select Mind Trick for your Force weapon. When he shoots his lightsaber, block it away to the ground. Use the Jedi Mind Trick and attack him. He will be vulnerable for about five seconds.