Nega Elemental is a special monster in Final Fantasy X bred in the Species Conquest section of the Monster Arena after three of each of the Elemental species fiends throughout Spira are captured. It shares its model with the Dark Element.
Nega Elemental absorbs all elements, including Holy, and will counter all attacks, abilities, and Overdrives with Ultima. At the start of the battle, it will cast Reflect on itself and then bounce a variety of strong magic into the party.
The best strategies to defeat it are much the same as for the Jumbo Flan: Doublecasting Ultima and using Copycat. In a pinch, Anima can be brought in to use Pain, provided she is strong enough to withstand Ultima. Using Anima, one will need to ensure to cast Reflect summoning her, as Nega Elemental will use Osmose and steal her MP.