You can get The End relatively easily by popping open the disc cover just before an enemy's about to cast a spell. Spell animations have to load from the disc, so nothing'll happen. Then select Selphie, who should have low HP or Aura by the way, and start hammering 'O'. Be careful not to skip The End by mistake, or else you'll be "mad as a bug". Once The End comes up, unleash it at thine enemy who, being naughty in my sight, shall snuff it.

This even works for Ultimecia's final form, although The End won't defeat her outright - but it will take you right to the point where she starts to speak - "Reflect on your... childhood..."