The end boss is only vulnerable when his protective bubble is removed by pulling him into the moving purple areas.
He will also apply a group-wide Daze
if he hits anyone with the third hit of his auto-attack chain. Avoiding this is the highest priority during the fight. Either use blocks, dodges and skills that make you invulnerable or - most commonly - bring permanent Stability
. It may also be avoided by not standing in front of him when his attack hits. Learn to recognize the animation.
By bringing permanent Stability
and reflects, enough DPS or a dedicated healer the fight can be easily cheesed.
Note that you can take aggro from him by walking inside his hitbox and cancel the third auto-attack by moving out before.
Another trick is to bypass the damage from purple zones by repeated jumping.
Also pay attention to the small JT-12
golems and their AoEs as they apply Knockback
and can shove you off the platform.