Sephiroth's Power
Bolt3-All, Fire3-All, Ice3-All, Massamune slash, Shadow Flare, Stigma, Pale Horse(status changer and damage), Demi3, Supernova, Bizarro Energy(restorative magic in Bizarro Sephiroth form), Wall No Holy(obviously), Ultima(I'm pretty sure) or Flare in his arsenal. Ultimately, Shadow Flare is far more powerful.

I don't see it. Sephiroth needs time to cast all these spells, but in battle, there is nowhere to run. You have to stay there and get hit by it. Bowser is not restricted by these laws. While Seph needs the time to gather his strengh(even if its only for 3 seconds) Bowser can simply move. Despite his size, his power allows him to move a long distance in one very high jump. The impact of him hitting the ground would undoubtedly shake it and Seph up a bit. The distance between them could be covered by Bowser in seconds. Seph mayb ecould get off one spell before Bowser reached him. And then what? Would one Ice3 spell kill Bowser? Could Stigma destroy the giant that is Bowser? Not likely.

After that one spell attempt by Seph, Bowser to knock his head in. Or, breath those 3, short fire breathes that produce 3 fireballs that proceed to follow the person there aimed at until they hit, or are deflected some how.

Wall would be advantagous to Seph. A physical and magic barrier is a good trick. But, it doesn't save him from getting hurt some, like a bulletproof vest. Bowser could knock away the area Seph is standing upon, and Seph could attempt to fly away. However, ojne quck swhipe of Bowser's arm or tail, or even a fire blast could send Seph flying backward. He may not be killed by the blow, but it would cause him some damage as well as getting him off his feet.

Sephiroth would need his speed and strength for this battle. A magic approach isn't reasonable. Sephirtoh's physical power is alot more impressive than his magic. Just fly around and swing his sword. That's also a simple plan, but like my plan for Bowser, it is very effective.