The Phoenix is one of the four Guardians of the Void. The name is a certain giveaway for a fiery encounter, and that is exactly what you can expect when you take on The Phoenix. Unlike poe items, the Phoenix deals massive amounts of Fire Damage with his attacks, some of which can Ignite. He's immune to Knockback, Ignite, Chill, and Freeze.
His attacks break down as follows:
Melee Attack: Deals Fire Damage and can Ignite.
Whirling Charge: Locks onto a target, charges up, and spins to the target's original location, dealing Fire Damage multiple times throughout the attack's path.
Fire Bomb: the Phoenix brings his swords down into the ground, emitting a debuff that reduces the player's Fire Resistance by -50% before channeling to release a large Fire discharge.
Resistance Debuff: For every 10% of Life that The Phoenix loses, the player's maximum Fire Resistance is reduced by 1%.
Add Spawn: For every 10% of Life that The Phoenix loses, a phoenix will shoot fire projectiles at the player (and will respawn a few seconds after dying).
Fire Spout: For every 10% of Life that The Phoenix loses, a fire spout deals Fire Damage in a small AoE as it travels around the arena (which lasts for a few seconds and are then replaced by another).
Unlike the other fights, you're safe to keep your standard engagement ranges throughout this one. Don't worry about being too close to The Phoenix. When the Phoenix goes to perform a Whirling Charge, be prepared to dodge left or right (or even behind the Phoenix) as quickly as you can. This attack can be devastating if you take a direct hit because it can damage you multiple times.
You'll have to prepare yourself for adds spawning in, so dispatch of these when the right moment arises. One particular opportunity to do so is when The Phoenix begins to channel Fire Bomb, so in this instance, you'll want to move away and take out anything else knocking around in the process.
As The Phoenix begins to channel Fire Bomb, move as far away from him as possible. During that time, you can kill off some of the adds that have spawned. When The Phoenix is around 50% Life, you want to be sure you're utilizing a Ruby Flask, because you're going to be passively losing Fire Resistance as his health gets lower and lower.