Since Cyborg, Starfire and Beast Boy are protected to Trigon's powers thanks to the Ring of Azar, he refuses to expend more of his energy to destroy them and uses his powers to create evil clones out of each of them and pits them against each other, each against his or her own clone.
Since she is a clone of her, Nega Starfire is evenly matched with Starfire and is aware of everything about her, including her inner feelings and she uses them against her. During the battle, Nega Starfire taunts Starfire for her feelings for Robin and says that when she wins Robin will be hers.
Later, as the tide of the battle above ground starts to turn against the Teen Titans, they decide to switch combatants in hopes of regaining the advantage. Therefore, Beast Boy fights against Nega Starfire and manages to beat her by turning into an Apatosaurus.