[q=Hyperion4444]very few bosses go up to 100, but they are all optional and not all of them either.[/q]Not quite right... plenty of bosses can attain the maximum level, and many of them are disproportionately powerful as a result - i.e, they become stronger at a much faster rate than your own characters do, as they level up. If you fight Raijin, Fujin and Seifer at Lunatic Pandora with a party who're all at level 100, those three will be at level 100 too - and each will have about 40,000 HP. That's more than your entire party gets. By keeping the party at a low level, the bosses remain suitably weak; but it's still easy to get your party up near their maximum by using strategic Junctions.

Sure Ultimecia can blow away magic spell; but if a character's carrying a lot of different spells, her chances of hitting a crucial spell are pretty low. Out of all the times I've beaten FFVIII, only once did she blow away the Full-Life spells I was using for someone's HP junction.

Sephiroth certainly does level up as you do in FFVII, but his incremental increases in strength are nothing compared to Ultimecia's.