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 physical and special damage attacks with Ultima. It will drain a party member's HP or MP, or cast Reflect on itself and then bounce a variety of strong spells onto 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 cast Reflect after summoning her, as Nega Elemental will use Osmose and steal her MP.