Okay, you've made it through the
Domination,
Deathmatch,
Assault &
CTF levels. You've beaten the first three Final Challenge levels and now it's time to fight
Xan. Should you be having some trouble at this point, here is some help.
When the level starts grab the rocket launcher or the flak cannon and run to the horseshoe shaped passage, picking up the health vials and the shield belt. Just outside the horseshoe passage there is a another passage going down, grab the armor at the bottom of it. After that, when Xan comes near you, run backwards, shooting either grenades or flak shells at him. Make sure not to run out of health or ammo (if you get low, make a run for more.) The most important thing to remember, is to keep grabbing the shield belt and armor, as if he gets both, it's incredibly hard to kill him. Oh, and try not to fall off into space too often.