If you want Ultimecia's Castle to be super-easy, work on your junctions, fight Ultima Weapon, and get Eden. He does a ridiculous amount of damage to everyone--at times over 100,000, IIRC. Plus, you can draw all the Ultima magic you'll need from Ultima Weapon. Very useful. And be sure to fight the Tri-Face fixed encounters in the deep sea center (they disappear after you defeat Ultima, so do them before) to collect plenty of curse spikes. Curse spikes --> dark matter --> shaman stone --> rosetta stone = win.