At first, when I thought of this song in Advent Children, the scene that came into my head was more the motor chasing scene, rather than the Sephiroth battle. But maybe I'm just used to One Winged Angel too much? Hehe.
Overall, a cool piece. And as others have said, slightly repetitive, but in boss battles, people tend to focus a lot more on the gameplay than the music, so this is not a major issue. I would also go for choir sounds, especially if you had Sephiroth, or a similar character for that matter, in mind. It would also help to break through the monotony, I think.