Note to all:

ULTIMECIA'S LEVEL IS THE SAME AS YOUR PARTY'S LEVEL, just like (nearly) all FFVIII enemies.
So if you fight her with characters who're at a lower level, she'll be at a lower level too. With less HP, weaker magic, inferior strategies. The secret to an easy Ultimecia fight is to keep everyone at as low a level as possible, and use junctioning to make them stronger. It's easy to have a low-level party (less than level 20) who've got maximum HP, strength, magic, etc.