Ultimecia will randomly select three characters to battle, and if any character is KOed and left alone for too long, she will absorb the character into time, permanently removing them from the battlefield and replacing them with a new character.
Ultimecia possesses a wide range of spells that can inflict various status effects, such as Maelstrom which reduces the party's HP by 66% and inflicts the Curse status. She can cast Double on herself. If a Guardian Force is summoned into battle, she will eliminate it immediately.
After defeating her first form, Ultimecia will summon Griever into battle.
The player can junction the three characters they wish to use in battle with all the Guardian Forces and magic they'll need, rather than junctioning all six. That way, Ultimecia can eliminate the unjunctioned characters without loss and bring in the prepared party. Alternatively, one can customize their GFs until all six characters can be equipped with the full range of desired abilities.