Whittle Jerec's health down with your lightsaber. Guns don't work on him, as he just pulls them out of your hands. Force Absorb, if you have it, is a great power to use against him as it will block any Dark Side Force power Jerec may use. Try to stay close to him. If you get far away, Jerec uses Force Destruction on you. (believe me, it isn't pretty) If you're low on health, however, run quickly towards one of the two recessed areas. One has a bacta tank, the other has a health pack. Once you get those, run back and enter the fray. When Jerec is badly hurt, he'll ride the air lift in the center and will start to draw upon the power of the Valley of the Jedi to heal himself. Don't follow Jerec up. Instead, run over to one of the statues. As Jerec starts to heal, the statues will start to move towards the center. You have to be quick! Use Force Speed to run behind both statues and hit a button there. When both buttons are pushed, the healing process stops for Jerec. If done quickly enough, Jerec will be weaker. If you're not quick enough, Jerec will laugh and he'll be back to full strength.
Once you've weakened him, go back to fighting Jerec. Repeat all the steps mentioned above until you eventually wear him down and defeat him.