Phoenix will start the fight with a buff that increases fire attack. Its second turn, it will do another buff that will give itself reraise, increase healing power, debuff res, and all elemental resistances, becoming 19% all element resist except for water element staying at -10%.
Phoenix often uses Fire Breath, dealing fire damage in an area. It can also use Shock Wave, which is another area damage, but it has a chance to stun.
You can use VCs like Death Machine, or Netherworldly Crimson Orb, Bloody Moon for some area attack resistance. The fight becomes much easier once you have enough survivability until your healer gets their turn.
When Phoenix's HP gets low, usually after its reraise activates, it can heal itself to full. Try to burst it down when it's low to prevent this from happening.
Recommended Units
Having a tank can help make Phoenix target attacks to a different direction so the area attacks won't hit your team, which really helps if your team isn't fully levelled yet, but the fight is still quite doable by going full damage if your have enough of it.
Any unit that can provide quick group heals are good, especially if they can deal a lot of damage as well. Ildyra is pretty suited for this fight since she's a water element unit, meaning higher fire resist and can use water attacks.
Most damage dealers are still doable, even after Phoenix uses all elemental resist since it's not that high, but water units are the best ones to use.
Tidus helps with getting the 3 elemental chain all by himself through his burst. He can also inflict blind to Phoenix from Samurai subjob's Nightveil.
Creysse has Esper killer buff in an area, and she can attack safely from afar.
Glaciela won't be doing high damage since Phoenix has pierce resist, but she can use strike attacks, or just be useful for elemental chains to clear one of the missions.
Zuu VC and Odin esper has avian killer if you are lacking some damage.
Any strong magic damage dealer like Sakura or Garvall can still work since it's slightly weak to magic.