Use the fast saber stlye on Desann when you finally reach him--unless you're REALLY well practiced on multiplayer mode at jedi knight difficulty--possibly jedi master if you have the gumption.
Do try to push the switches on the walls to activate the force beam for a few seconds--Desann will stay, still staring and tauting. Then dorp in and shove him over or wait for him to come after you (Do try to draw him AWAY from the pillar of light or else he will get the power up and you won't!) and then go for it. It will buy you a precious 10 seconds or so of decreased vulnearability and being powered up--your force powers will have more stamina and kyle will be more resillient.
It's a tough fight--even on medium difficulty. I remember I finished him off in a really unique way; I either pushed or pulled him down on the altar so he was raised up ay my swinging level, went quickly around to a side of him and swung the saber from my right to left and cut him vertically from either toe to head or vice-versa from the side! That hit killed him...I only wish I had dismemberment enabled! That would have looked awesome! It would have been even better if I had a way of recording it and had done so--I'd broadcast it all over the web!
--The voice of Luke is remarkably like Mark Hamill's.