Sephiroth's so badass AND he so totally knows it.
It even has a new version of "One-Winged Angel" toward the end, and we all know what THAT means, right?
EDIT: Here's a translation of the scene, from Chocolinx's translation at the Advent Children Crisis Core Forums (just follow the instructions to access the Spoiler Forum).
Genesis and Angeal sit side by side on top of the Junon Cannon. Genesis reads his favourite verse from Loveless out loud.
Genesis: 「The most profound gift is given by the Goddess herself and in pursuit of this gift we set on a journey and take flight. Hopeless as it may seem we stand and carry onward with the ripples of water that surface on our hearts.」
Sephiroth: 「Loveless」 Act 1
Genesis: Sad isn't?
Sephiroth: Having you read it to me, makes me remember some awful things.
Angeal: Don't go dropping your sword because you're too scared.
Genesis: I wonder which one you're talking about?
Sephiroth: Looks like you need to brush up your sword skills.
Angeal: As expect of Sephiroth huh?
Genesis: Angeal, stand back please. I'd like to duel Sephiroth alone.
Genesis: I too would like to become a hero.
Sephiroth: Very well.
Genesis: When will you stop acting so relaxed?.
Genesis gets the jump on Sephiroth and starts casting multiple spells. Genesis skillfully controls his firas and directs them at Sephiroth. It appears Sephiroth has taken a full blow at his Firas and in the smoke Sephiroth is no where to be seen.
Angeal: Stop this at once. Do you plan on having this place destroyed?
Genesis: All I want to be is a hero!
Genesis sticks his palm in Angeal's face and starts casting a spell! Angeal takes a full blow at it! Suddenly Sephiroth breaks himself free from Genesis' magic. Now Sephiroth is on the push and Genesis is being pushed back. Sephiroth with many quick swings of his swords ends up destroying the Jeuno Cannon. Angeal jumps in and tries one more to stop the two.
Angeal: That's enough!!
Genesis: You're in my way!
Genesis attempts to break through Angeal by casting another spell, but ends up breaking Angeal's sword. The shard of the sword ends up hitting Genesis on the shoulder and the VR training ends.
Genesis: It's just a small scratch. If I just leave it alone it'll quickly heal. 「We still have our promise for tomorrow. Let us return to the place where you first came from.」