Where does it say it's a level 99 party anyway? (I don't see it, so..) I always use HP/MP Plus Materia Mastered to max out HP and MP before level 99 anyway, so that doesn't mean that they're at level 99. I'm still having a hard time understanding why you don't understand. Like I said, I've killed Seph with Omnislash in one shot as well.

Edit: The video does load, it just takes awhile. Put the progress bar of the video back, and it'll load a little more.