Viper Napuatzi - Bugged boss

The boss misses a certain order of attacks/phases, it´s so chaotic. Once I got the poison spears 3 times in a row, like right after each other. How are you suppossed to damage the boss when she is invincible all the time.
The bug part is also very funny, every time the arena shrinks and the soldiers move forward, there is a chance that the boss buggs itself into the soldiers and becomes undamagable, just standing there smiling at you knowing, you have to reload the fight.