Get Steiner with Shock. I think you need the Ragnorok for that one, which I think you get from the Chocobo sidequest- do it, it's worth it.

Then catch 99 frogs at the various marshes and use Quina's frog drop.

Defeat a bunch of the grand dragons with Freya in your party, then use her Dragon Crest.

Just go around and steal a bunch of stuff with Zidane. Raise you spirit stat as much as you can to increase the likely-hood of success.

All these attacks will do 9999 every time. It's the best offensive attack you can have. Sometimes I replace Quina with a healer, or Amarant with the healer ability equipped, but with them, you should mash Necron into the ground. Sure, it takes a lot of time to kill enough dragons for Freya's Dragon Crest to max out, and frog catching gets very tedious, and stealing with Zidane takes forever, but in the end, you can take everything down easily, even Ozma.

If you have auto-potion, chemist and auto-regen and plenty of stocked hi-potions, you won't need a healer either. Invincibility!