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.