Serah and Noel may be the main characters in Final Fantasy XIII-2, but Lightning is playable at some points in the game too. Back at E3, we saw Lightning battling Chaos Bahamut while riding Odin. Now, you can see about ten seconds of that scene thanks to Square Enix Europe.
This key visual shows the two girls (with transforming weapons) that defied the gods.
An event scene shows a grotesque creature emerging from the void in New Bodhum. The other screenshot shows Gran Pulse native Noel fighting enemies with his two swords.