You're thinking of Charon. Its on the Dark Knight DS, and will only kill the Black Elemental if the character's HP is above 5,000. (Charon damage = Character's max HP * 2)

Usually this boss has 9,999 HP, but when Oversouled it has 10,998 HP, meaing it cannot be killed in one hit unless you have an accessory that provides Break Damage Limit.

Unless you're playing a New Game+ you won't have the Cat Nip accessory yet, so that method cannot be used.

A good piece of advice is to use the Songstress' Magical Masque to negate all magic used by Black Elemental, meaning you are free to deplete its HP with attacks such as Darkness without the risk of being hit by Ultima.