Location: Igraine Shrine of the Fire Trial, to fight it, you need to gather other normin first. See THIS PAGE for the all normin location. After getting all of them, go to nearby save point.
There should be a skit to trigger. See the skit and then go to Igraine, shrine of the fire trial. Go to ritual room (boss room).
Recommended Level: 81 – 85
Recommended time frame: Before the final battle or after clearing the game.
Don’t laugh at his HP, he will resurrect every time he dies (4 times on hard difficulty, not sure about other difficulties).
You definitely need to armatize on this battle. Armatize with Mikleo, because the form is very safe from the Phoenix attack. Your attack pattern should be
Side step/step back/ dash forward => Blistering Fang => Binding Vortex
Why not attack on the first attack of the combo?
Because the normin will resist against that attack, and if he goes toward you then he will reach your spot by the time you finish the attack. Blistering Fang will stagger the Phoenix because it’s a power hit. That’s why I suggest you don’t attack at the 1st combo. Let Blistering Fang do its job :).
 Side step/ Step Back if the Phoenix charge towards you (order your partner to attack him if he charge toward you)
 Dash Forward if the phoenix goes toward your friend so your blistering fang can reach him.
Set your partner to “prioritize defense” so they will make the phoenix chase them around (make sure your armatize them, phoenix can one hit KO the character that on “normal” state).
Item that can be useful on this battle:
Arcane Bottle/Elixir: You can buy these items when the shop’s star level are above 3.
Grape Gel: Heal 70% HP, useful to heal after guarding for a long period.
You need to use mystic arte if you want to end this battle quickly, that’s why I suggest you to restock arcane bottle (elixir if you’re rich; use it only on emergency situation). Make sure you see my video above to see what I meant on this post :). You may die few times until you can execute this plan perfectly.
Good luck and please share if you have other tips to defeat him.