Sephiroth after failing to defeat Cloud comes to the world of Marvel in which he is able to obtain the Heart Of The Universe and Infinity Gauntlet, and he gains full knowledge thanks to his experiences of being in the life stream. Lucifer also appears from the DC universe and ends up in the Marvel Universe. Who will win? The One Winged Angel or the Fallen Angel?
Both are at full power
Sephiroth gets one day of prep and has access to all of his abilities along with the HOTU, and Infinity Gauntlet
Bonus: Dialogue can also be included as I think this would be an interesting crossover battle of the angels.