I'm not terribly happy with this fic, but I guess it's good enough.
Eiko could tell they were losing. She and Vivi were running out of energy – she barely had enough mana for another Cure spell. Quina was unconscious, lying face-down on the hard wooden platform. Dagger was barely standing, and Zidane was badly hurt from trying to protect her. Meanwhile, the monster at the Iifa Tree's heart seemed to barely be damaged by their efforts.
It seemed hopeless. The monster continued to taunt them as they struggled against it. It sent a storm of sharp leaves out to attack the group. Zidane managed to pull Dagger out of the way in time, but Vivi and Eiko weren't so lucky. Eiko was knocked clear across the platform. She didn't have time to see what had happened to Vivi.
The scene before her started to blur. She reached into her pocket, desperately trying to find some kind of potion. She found nothing.
She grew dizzy, and struggled to stand. Somewhere, she heard Zidane screaming. She couldn't see why, but it couldn't be good.
It couldn't end here! They still had important things to do! She'd just met these people – it was too early to lose! She tried to stay standing, but she just couldn't. She collapsed.
It was hopeless. She was only six, and she was going to die. They were all going to die.
As her awareness began to fade, she felt a strange warmth from somewhere nearby. She was too exhausted to wonder why, but it did feel kind of nice.
After a while, her thoughts began to clear, and she realized that her head felt funny. It was kind of like how she felt after summoning an Eidolon. But that couldn't be right. Her mana was nearly gone, and she always needed a lot for summoning.
She opened her eyes, feeling her energy returning. In front of her was a huge, beautiful bird. It was the color of a brightly burning flame, with its wings alight in every color imaginable. The heat it gave off was not painful. Instead, it gave her strength. Her wounds were gone, and she felt no pain. Slowly, she stood up. Nearby, her friends did the same.
Eiko remembered a colorful feather she'd found on the way here. It had power similar to her stones, but she hadn't felt the presence of a new Eidolon. Right then, she remembered something she'd been told a long time ago. It was a story about an Eidolon that sometimes came to the aid of Summoners, provided they were lucky enough to find one of its feathers. They'd give the Summoner strength in their darkest hour, and allow them to fight on.
Its name was Phoenix, the bird of fire.
They won the battle that time. When they finally found their way out of the tree, Zidane approached her. "Hey Eiko, did you do that" he asked. "Dagger says she didn't, and I don't know who else could have summoned it.."
"I don't know," Eiko admitted. "Guess we're just really lucky."
They're fighting Soulcage because that was the only time Phoenix ever showed up in my game. At least, I think that's when it was. It's been a while since I played FFIX, so I can't say for sure.