Xigbar is the easiest of the Data Battles, when he starts shooting red bullets just Reflect like a madman, after 2-3 rounds of reflecting his bullets he will be in a weak state which let's you attack him, go aerial and do Horizontal Slash(that skill that is done with square in the air) all you can and don't even use a Finisher, for some reason that makes him not change the stage much or at least he didn't with me, when he goes into FPS mode just move around and mash triangle.
If he changes the stage into the small one in which he's shooting like crazy just go into Limit form and do some of the long animations, or actually learn where to move(You have to glide around to avoid his crap and then dodge for the other one, kinda difficult to explain).