DLC only. You get a RAV version of her from the Lightning & Amodar DLC and a COM version from the Requiem of the Goddess one. Though you won't be strong enough to take on the L&A battle until chapter three or four at least, and even then you might need to fight them numerous time to get her, since you need to get her crystal just like a regular monster's.
You're guaranteed the COM version upon completing RotG (and she's easily the best COM in the game), but it has major spoilers for the end of the game. You could probably just skip all the cutscenes though and not have much spoiled. --- Monkey see, monkey do - just another animal in the zoo.
srry i just bought the game so i would have to download it. then beat her at the arena then hope i get the monster gem thing from her and then i unlock her correct?
For Requiem-Lightning, all you need to do is to get a five star ranking in both of her battles. It's not that hard on a proper level and the right strategy. Of course, Requiem also involves ending spoilers. Be prepared or skip the cutscenes, if that's possible. Lightning-Ravager from the arena is beatable around chapter three or four, if you developed Serah and Noel properly. (Meaning: Doing sidequests as well.) Oh, and there's a fragment skill that raises monster drops. I misplaced my strategy guide again, but I believe you need to do all sidequests at both "Vile Peaks"-Areas.