My imagination ran away with me and caused me to come up with a bunch of potential Sephiroth mechanics...

1: Auto-attacks that cause stacking Max HP Down, combined with tankbusters that causes incurable. Frequent enmity juggling between the tanks is a neccesity.

2: A variant of Heartless Angel that is simultaneously weaker and stronger, weaker in that it can be interrupted by doing enough burst damage during the cast, thus avoiding it all-together, but stronger in the sense that if it goes through it will nuke your MP as well...at which point you are probably screwed. So basically a DPS check or end up with impotent casters and healers on top of having only 1 HP.

3: Pale Horse, straight line AOE that doesn't do very much damage, but inflicts a nasty transmutation effect on anyone hit. Have fun being a toad.

4: Supernova, self-explanatory.

5: Meteors, lots of meteors, regular AOE's, proximity AOE's, Meteor circles, some of which overlap each other.

6: Fire Pillars, as in his KH2 appearance, point blank AOE with draw-in effects. Combine with proximity meteors along the edges of the arena for extra fun.

7: Shadow Flare orbs, they have appeared before, during the Diabolos fight at the end of Dun Scaith as one example. Except this version has homing properties instead of just traveling in a straight line. They go right for whatever player is closest, forcing you to keep moving or get blasted.

8: Divebombs, he teleports to the edge of the arena and flies across at high speed, cleaving everyone in his path for massive damage.