I wonder... I have no real favorites here. Bowser, I guess I'll stick with.

So, lava. This is like the second time you fight him in Mario 64. Sephiroth can do more than grab Bowser's tail, but Bowser's power tactics can help him here.

He needs only jump, or perhaps just use his tail and the arena would start rolling around or perhaps breaking off and falling into the magma. Sure, Sephiroth could simply levitate, but then he's open for a giant burst of fire from Bowser's mouth. Sephiroth needs time to fly at Bowser to hit him with his Massamune, or he needs time to focus his magic. Bowser would use this time to simply do the actions listed above.

Perhaps, Sephiroth could just start by flying as Bowser, without standing on the rock. However, Bowser could simply charge. Or, breathe fire. A full attack by Bowser would be too much for Seph to take. Breathing fire would cause Seph to dodge and take time
to finally reach Bowser. Even then, couldn't Bowser simply reach out and swat the pesky fly?

Also, couldn't he teleport? Been along time since I played it and I didn't like it too much, so...