Try this. Put the level as a mod for Jedi Knight. Then you can play it in singleplayer. Put on whiteflag 1, and get all weapons. Go to the ghosts room and aim. Put heavy object on mouse button. Watch fireworks. Fireworks run out. Get all weapons. Watch fireworks. Fireworks run out. Get all weapons. Watch fireworks. Fireworks run out. Get all weapons. Watch fireworks. Fireworks run out. Get all weapons. Watch fireworks. Fireworks run out. This way, you don't actually have to sit there and try to fight the un-fightable. When one weapon runs out, it switches to another (check the box in settings for no dangerous weapons so the ghost isn't moved). I would try this, but I regard it as a waste of time after I realized how much health the thing has. Oh, and by the way, I opened the GOB file and found a "ghostdie.wav" file in it...
"You're only supposed to blow the bloody doors off!" Anyone who recognizes this quote is awsome.