STAR WARS Jedi Knight II: Jedi Outcast

This is story-related and cannot be missed. This trophy will pop at the end of Mission 24 – Yavin Final after defeating Desann.

Tips on how to defeat Desann:
 

Desann is the toughest enemy you’ll face. His lightsaber swings will be really strong, and if you use an offensive Force Power, he’ll copy you; however, Force Lightning is again the best thing to use against him, but Force Heal should also be something you rely on. There are some shield and health packs up above on the outer ledges of the room, so make sure to utilize those if you need to for emergencies.

Now, if you are not great at lightsaber battles, there is a saving grace available that was hinted at by the cutscene before the fight. Instead of going down to face him, go up the path to the right or left and you’ll see a Force Pull panel. Use Force Pull to activate the panel, then go to the other side of the room and do the same. Once both panels are activated, the pillar in the middle of the room will activate the special beam every minute or so. Once you touch it, you’ll be momentarily invincible. Use this time to attack! Be aware that the effect will dissipate, and you’ll be vulnerable again. Run around the room until you can see the beam activate again. Rinse & repeat.

One quick warning, if you don't touch the beam first, Desann might jump into it and become temporarily invincible himself!