Rathbreaker is broken

Rathbreaker, the first boss in Act 2, will stand still and takes no damage. He becomes invincible so I have to leave the Act or die to try him again. This happens more than it does not.

This seems to still be a common problem with Rathbreaker. It may happen more often when you leave the hill but it can still happen when you stay right next to him. One option you have is to restart the fight by choosing Restart at Checkpoint from the ESC menu (press Escape or the suitable controller menu button).

However, please note that Path of Exile 2 has its own Bug Report Forum for early access.