Yuna has considerably fewer hit points than Valefor and can cast supportive abilities like Cure and Protect, so you should focus your attacks on her after toppling their stack. Once Yuna is knocked out, the battle becomes much more manageable. Most wind-based Mirages like Valefor are weak to ice damage, so the Blizzard spell is handy for dishing out damage (if you have a bablizz Mirage from defeating Shiva in the Nether Nebula, for instance). If you purchased the Warrior of Light Champion Medal, you can unleash a devastating amount of damage while boosting your defense.