I would do everything Sephiroth did, but better. When killing Aeris, I would actually notice the White Materia she wears in her hair, and destroy it before Cloud and his friends could use it to stop Meteor. Then, instead of constantly hiding behind Jenova, I would actually kill the heroes myself with Jenova as a backup in case they were able to overwhelm me with numbers. Also, since I'm technically in the Lifestream anyway, why not just use that to my advantage. Theoretically, I could probably control it, make it rise to the surface of the Planet, and give everyone Mako poisoning to put them in a vegetative state. Where I'm going with all this is, I would be Sephiroth, but emulate Kefka and actually destroy the world.