The biggest tip I can give for beating Xan is the following: do not let him have the shieldbelt under any circumstances. Sacrifice a life if you have to, if it will lure him away from the belt. If he gets it, expect to get fragged at least three times while you wear it away, and that's if you're lucky and he doesn't get a refill on it.
The only other advice I can give is Xan's weakness: the BioRifle. A fully-charged wad will instantly paste him to the floor, even if he's at max health and armor with the shieldbelt. Naturally, of course, this is also a very dangerous situation for you, but there's nothing for that. Just be careful: you'll only get one shot at this, because if you miss he'll have already switched to his Flak cannon, and will be in prime position to turn you into a fine red mist.