The Arbiter of Ash starts by POE2 Currency firing a beam of fire down the center of the arena. This attack is easy to avoid by staying to the sides of the arena.
He will also shoot fire projectiles that spread out in different directions. These projectiles can be dodged if you stay alert.
Firebomb Phase:

The Arbiter will dash around the arena, dropping firebombs that descend slowly to the ground. To avoid them, you must move to the circle directly beneath each bomb.
The bombs explode after a short delay, so be sure to quickly move from one safe zone to the next.
Second Phase (55% Health):

At around 55% health, the Arbiter pulls a massive sword from the center of the arena, and the battle intensifies.
He introduces a new firebomb mechanic, where a bomb is dropped in a safe area, but a wide beam of fire fills the rest of the platform. You’ll need to jump into the safe area quickly before the bomb explodes, and then dodge out before a slam attack follows.
Wave of Fire Phase:

The Arbiter summons huge waves of fire that will fill most of the arena. There will be only a narrow lane marked with blue lines that you must navigate to avoid being one-shotted.
The direction of the firewaves will change, so you’ll need to adjust your positioning quickly.
Frost and Ice Phase:

The Arbiter summons two orbs—one blue and one purple. The blue orb will freeze the entire arena in frost, slowing you down. You must then walk over the purple orb to banish the ice from the platform.
During this phase, the Arbiter will continue attacking while you're dealing with the chill, so you need to be quick on your feet.
How to Defeat the Arbiter of Ash:
To defeat The Arbiter of Ash, you’ll need a combination of speed, precision, and mobility. The following are key strategies to win:

Maximize Movement Speed: Due to the intense mechanics, you must be able to move quickly to dodge attacks and reach safe zones.
Increase Maximum Life & Energy Shield: The Arbiter’s attacks are brutal, so having a large health pool helps you survive longer in the fight.
Dodge and Evasion: Using Acrobatics or evasion gear will help you avoid some of the Arbiter’s deadly attacks.
Stack Fire Resistance: Most of his attacks are fire-based, so increasing your fire resistance (even beyond the normal cap) will reduce incoming damage.
Understand the Patterns: Learning the Arbiter’s attack patterns is essential for success. Practice dodging and positioning to anticipate his moves.
Use the Right Gems & Buffs: Buffs that enhance your mobility, resistance, and healing will significantly improve your chances.
Rewards for Defeating The Arbiter of Ash:
Upon successfully defeating The Arbiter of Ash, you will receive:

Book of Knowledge: A special item that unlocks new passive skill tree nodes.
High-Quality Randomized Loot: The Arbiter drops powerful gear, including unique items that are crucial for endgame builds.
Achievement Rewards: Special achievements, cosmetic rewards, and titles for POE 2 Currency completing the most difficult encounter in the game.